diff --git a/src/infrastructure/visitor.py b/src/infrastructure/visitor.py
new file mode 100644
index 00000000..78c0ca45
--- /dev/null
+++ b/src/infrastructure/visitor.py
@@ -0,0 +1,166 @@
+"""Visitor state tracking for the Matrix frontend.
+
+Tracks active visitors as they connect and move around the 3D world,
+and provides serialization for Matrix protocol broadcast messages.
+"""
+
+import time
+from dataclasses import dataclass, field
+from datetime import UTC, datetime
+
+
+@dataclass
+class VisitorState:
+    """State for a single visitor in the Matrix.
+
+    Attributes
+    ----------
+    visitor_id: Unique identifier for the visitor (client ID).
+    display_name: Human-readable name shown above the visitor.
+    position: 3D coordinates (x, y, z) in the world.
+    rotation: Rotation angle in degrees (0-360).
+    connected_at: ISO timestamp when the visitor connected.
+    """
+
+    visitor_id: str
+    display_name: str = ""
+    position: dict[str, float] = field(default_factory=lambda: {"x": 0.0, "y": 0.0, "z": 0.0})
+    rotation: float = 0.0
+    connected_at: str = field(
+        default_factory=lambda: datetime.now(UTC).strftime("%Y-%m-%dT%H:%M:%SZ")
+    )
+
+    def __post_init__(self):
+        """Set display_name to visitor_id if not provided; copy position dict."""
+        if not self.display_name:
+            self.display_name = self.visitor_id
+        # Copy position to avoid shared mutable state
+        self.position = dict(self.position)
+
+
+class VisitorRegistry:
+    """Registry of active visitors in the Matrix.
+
+    Thread-safe singleton pattern (Python GIL protects dict operations).
+    Used by the WebSocket layer to track and broadcast visitor positions.
+    """
+
+    _instance: "VisitorRegistry | None" = None
+
+    def __new__(cls) -> "VisitorRegistry":
+        """Singleton constructor."""
+        if cls._instance is None:
+            cls._instance = super().__new__(cls)
+            cls._instance._visitors: dict[str, VisitorState] = {}
+        return cls._instance
+
+    def add(
+        self, visitor_id: str, display_name: str = "", position: dict | None = None
+    ) -> VisitorState:
+        """Add a new visitor to the registry.
+
+        Parameters
+        ----------
+        visitor_id: Unique identifier for the visitor.
+        display_name: Optional display name (defaults to visitor_id).
+        position: Optional initial position (defaults to origin).
+
+        Returns
+        -------
+        The newly created VisitorState.
+        """
+        visitor = VisitorState(
+            visitor_id=visitor_id,
+            display_name=display_name,
+            position=position if position else {"x": 0.0, "y": 0.0, "z": 0.0},
+        )
+        self._visitors[visitor_id] = visitor
+        return visitor
+
+    def remove(self, visitor_id: str) -> bool:
+        """Remove a visitor from the registry.
+
+        Parameters
+        ----------
+        visitor_id: The visitor to remove.
+
+        Returns
+        -------
+        True if the visitor was found and removed, False otherwise.
+        """
+        if visitor_id in self._visitors:
+            del self._visitors[visitor_id]
+            return True
+        return False
+
+    def update_position(
+        self,
+        visitor_id: str,
+        position: dict[str, float],
+        rotation: float | None = None,
+    ) -> bool:
+        """Update a visitor's position and rotation.
+
+        Parameters
+        ----------
+        visitor_id: The visitor to update.
+        position: New 3D coordinates (x, y, z).
+        rotation: Optional new rotation angle.
+
+        Returns
+        -------
+        True if the visitor was found and updated, False otherwise.
+        """
+        if visitor_id not in self._visitors:
+            return False
+
+        self._visitors[visitor_id].position = position
+        if rotation is not None:
+            self._visitors[visitor_id].rotation = rotation
+        return True
+
+    def get(self, visitor_id: str) -> VisitorState | None:
+        """Get a single visitor's state.
+
+        Parameters
+        ----------
+        visitor_id: The visitor to retrieve.
+
+        Returns
+        -------
+        The VisitorState if found, None otherwise.
+        """
+        return self._visitors.get(visitor_id)
+
+    def get_all(self) -> list[dict]:
+        """Get all active visitors as Matrix protocol message dicts.
+
+        Returns
+        -------
+        List of visitor_state dicts ready for WebSocket broadcast.
+        Each dict has: type, visitor_id, data (with display_name,
+        position, rotation, connected_at), and ts.
+        """
+        now = int(time.time())
+        return [
+            {
+                "type": "visitor_state",
+                "visitor_id": v.visitor_id,
+                "data": {
+                    "display_name": v.display_name,
+                    "position": v.position,
+                    "rotation": v.rotation,
+                    "connected_at": v.connected_at,
+                },
+                "ts": now,
+            }
+            for v in self._visitors.values()
+        ]
+
+    def clear(self) -> None:
+        """Remove all visitors (useful for testing)."""
+        self._visitors.clear()
+
+    def __len__(self) -> int:
+        """Return the number of active visitors."""
+        return len(self._visitors)
diff --git a/tests/unit/test_visitor.py b/tests/unit/test_visitor.py
new file mode 100644
index 00000000..e6c282aa
--- /dev/null
+++ b/tests/unit/test_visitor.py
@@ -0,0 +1,367 @@
+"""Tests for infrastructure.visitor — visitor state tracking."""
+
+from unittest.mock import patch
+
+from infrastructure.visitor import VisitorRegistry, VisitorState
+
+# ---------------------------------------------------------------------------
+# VisitorState dataclass tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorState:
+    """Tests for the VisitorState dataclass."""
+
+    def test_defaults(self):
+        """VisitorState has correct defaults when only visitor_id is provided."""
+        v = VisitorState(visitor_id="v1")
+
+        assert v.visitor_id == "v1"
+        assert v.display_name == "v1"  # Defaults to visitor_id
+        assert v.position == {"x": 0.0, "y": 0.0, "z": 0.0}
+        assert v.rotation == 0.0
+        assert "T" in v.connected_at  # ISO format check
+
+    def test_custom_values(self):
+        """VisitorState accepts custom values for all fields."""
+        v = VisitorState(
+            visitor_id="v2",
+            display_name="Alice",
+            position={"x": 1.0, "y": 2.0, "z": 3.0},
+            rotation=90.0,
+            connected_at="2026-03-21T12:00:00Z",
+        )
+
+        assert v.visitor_id == "v2"
+        assert v.display_name == "Alice"
+        assert v.position == {"x": 1.0, "y": 2.0, "z": 3.0}
+        assert v.rotation == 90.0
+        assert v.connected_at == "2026-03-21T12:00:00Z"
+
+    def test_display_name_defaults_to_visitor_id(self):
+        """Empty display_name falls back to visitor_id."""
+        v = VisitorState(visitor_id="charlie", display_name="")
+        assert v.display_name == "charlie"
+
+    def test_position_is_copied_not_shared(self):
+        """Each VisitorState has its own position dict."""
+        pos = {"x": 1.0, "y": 2.0, "z": 3.0}
+        v1 = VisitorState(visitor_id="v1", position=pos)
+        v2 = VisitorState(visitor_id="v2", position=pos)
+
+        v1.position["x"] = 99.0
+        assert v2.position["x"] == 1.0  # v2 unchanged
+
+
+# ---------------------------------------------------------------------------
+# VisitorRegistry singleton tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorRegistrySingleton:
+    """Tests for the VisitorRegistry singleton behavior."""
+
+    def setup_method(self):
+        """Clear registry before each test."""
+        VisitorRegistry._instance = None
+
+    def teardown_method(self):
+        """Clean up after each test."""
+        VisitorRegistry._instance = None
+
+    def test_singleton_returns_same_instance(self):
+        """Multiple calls return the same registry object."""
+        r1 = VisitorRegistry()
+        r2 = VisitorRegistry()
+        assert r1 is r2
+
+    def test_singleton_shares_state(self):
+        """State is shared across all references to the singleton."""
+        r1 = VisitorRegistry()
+        r1.add("v1")
+
+        r2 = VisitorRegistry()
+        assert len(r2) == 1
+        assert r2.get("v1") is not None
+
+
+# ---------------------------------------------------------------------------
+# VisitorRegistry.add tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorRegistryAdd:
+    """Tests for VisitorRegistry.add()."""
+
+    def setup_method(self):
+        """Clear registry before each test."""
+        VisitorRegistry._instance = None
+        self.registry = VisitorRegistry()
+
+    def teardown_method(self):
+        """Clean up after each test."""
+        VisitorRegistry._instance = None
+
+    def test_add_returns_visitor_state(self):
+        """add() returns the created VisitorState."""
+        result = self.registry.add("v1")
+        assert isinstance(result, VisitorState)
+        assert result.visitor_id == "v1"
+
+    def test_add_with_display_name(self):
+        """add() accepts a custom display name."""
+        result = self.registry.add("v1", display_name="Alice")
+        assert result.display_name == "Alice"
+
+    def test_add_with_position(self):
+        """add() accepts an initial position."""
+        pos = {"x": 10.0, "y": 20.0, "z": 30.0}
+        result = self.registry.add("v1", position=pos)
+        assert result.position == pos
+
+    def test_add_increases_count(self):
+        """Each add increases the registry size."""
+        assert len(self.registry) == 0
+        self.registry.add("v1")
+        assert len(self.registry) == 1
+        self.registry.add("v2")
+        assert len(self.registry) == 2
+
+
+# ---------------------------------------------------------------------------
+# VisitorRegistry.remove tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorRegistryRemove:
+    """Tests for VisitorRegistry.remove()."""
+
+    def setup_method(self):
+        """Clear registry and add test visitors."""
+        VisitorRegistry._instance = None
+        self.registry = VisitorRegistry()
+        self.registry.add("v1")
+        self.registry.add("v2")
+
+    def teardown_method(self):
+        """Clean up after each test."""
+        VisitorRegistry._instance = None
+
+    def test_remove_existing_returns_true(self):
+        """Removing an existing visitor returns True."""
+        result = self.registry.remove("v1")
+        assert result is True
+        assert len(self.registry) == 1
+
+    def test_remove_nonexistent_returns_false(self):
+        """Removing a non-existent visitor returns False."""
+        result = self.registry.remove("unknown")
+        assert result is False
+        assert len(self.registry) == 2
+
+    def test_removes_correct_visitor(self):
+        """remove() only removes the specified visitor."""
+        self.registry.remove("v1")
+        assert self.registry.get("v1") is None
+        assert self.registry.get("v2") is not None
+
+
+# ---------------------------------------------------------------------------
+# VisitorRegistry.update_position tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorRegistryUpdatePosition:
+    """Tests for VisitorRegistry.update_position()."""
+
+    def setup_method(self):
+        """Clear registry and add test visitor."""
+        VisitorRegistry._instance = None
+        self.registry = VisitorRegistry()
+        self.registry.add("v1", position={"x": 0.0, "y": 0.0, "z": 0.0})
+
+    def teardown_method(self):
+        """Clean up after each test."""
+        VisitorRegistry._instance = None
+
+    def test_update_position_returns_true(self):
+        """update_position returns True for existing visitor."""
+        result = self.registry.update_position("v1", {"x": 1.0, "y": 2.0, "z": 3.0})
+        assert result is True
+
+    def test_update_position_returns_false_for_unknown(self):
+        """update_position returns False for non-existent visitor."""
+        result = self.registry.update_position("unknown", {"x": 1.0, "y": 2.0, "z": 3.0})
+        assert result is False
+
+    def test_update_position_changes_values(self):
+        """update_position updates the stored position."""
+        new_pos = {"x": 10.0, "y": 20.0, "z": 30.0}
+        self.registry.update_position("v1", new_pos)
+
+        visitor = self.registry.get("v1")
+        assert visitor.position == new_pos
+
+    def test_update_position_with_rotation(self):
+        """update_position can also update rotation."""
+        self.registry.update_position("v1", {"x": 1.0, "y": 0.0, "z": 0.0}, rotation=180.0)
+
+        visitor = self.registry.get("v1")
+        assert visitor.rotation == 180.0
+
+    def test_update_position_without_rotation_preserves_it(self):
+        """Calling update_position without rotation preserves existing rotation."""
+        self.registry.update_position("v1", {"x": 1.0, "y": 0.0, "z": 0.0}, rotation=90.0)
+        self.registry.update_position("v1", {"x": 2.0, "y": 0.0, "z": 0.0})
+
+        visitor = self.registry.get("v1")
+        assert visitor.rotation == 90.0
+
+
+# ---------------------------------------------------------------------------
+# VisitorRegistry.get tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorRegistryGet:
+    """Tests for VisitorRegistry.get()."""
+
+    def setup_method(self):
+        """Clear registry and add test visitor."""
+        VisitorRegistry._instance = None
+        self.registry = VisitorRegistry()
+        self.registry.add("v1", display_name="Alice")
+
+    def teardown_method(self):
+        """Clean up after each test."""
+        VisitorRegistry._instance = None
+
+    def test_get_existing_returns_visitor(self):
+        """get() returns VisitorState for existing visitor."""
+        result = self.registry.get("v1")
+        assert isinstance(result, VisitorState)
+        assert result.visitor_id == "v1"
+        assert result.display_name == "Alice"
+
+    def test_get_nonexistent_returns_none(self):
+        """get() returns None for non-existent visitor."""
+        result = self.registry.get("unknown")
+        assert result is None
+
+
+# ---------------------------------------------------------------------------
+# VisitorRegistry.get_all tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorRegistryGetAll:
+    """Tests for VisitorRegistry.get_all() — Matrix protocol format."""
+
+    def setup_method(self):
+        """Clear registry and add test visitors."""
+        VisitorRegistry._instance = None
+        self.registry = VisitorRegistry()
+        self.registry.add("v1", display_name="Alice", position={"x": 1.0, "y": 2.0, "z": 3.0})
+        self.registry.add("v2", display_name="Bob", position={"x": 4.0, "y": 5.0, "z": 6.0})
+
+    def teardown_method(self):
+        """Clean up after each test."""
+        VisitorRegistry._instance = None
+
+    def test_get_all_returns_list(self):
+        """get_all() returns a list."""
+        result = self.registry.get_all()
+        assert isinstance(result, list)
+        assert len(result) == 2
+
+    def test_get_all_format_has_required_fields(self):
+        """Each entry has type, visitor_id, data, and ts."""
+        result = self.registry.get_all()
+
+        for entry in result:
+            assert "type" in entry
+            assert "visitor_id" in entry
+            assert "data" in entry
+            assert "ts" in entry
+
+    def test_get_all_type_is_visitor_state(self):
+        """The type field is 'visitor_state'."""
+        result = self.registry.get_all()
+        assert all(entry["type"] == "visitor_state" for entry in result)
+
+    def test_get_all_data_has_required_fields(self):
+        """data dict contains display_name, position, rotation, connected_at."""
+        result = self.registry.get_all()
+
+        for entry in result:
+            data = entry["data"]
+            assert "display_name" in data
+            assert "position" in data
+            assert "rotation" in data
+            assert "connected_at" in data
+
+    def test_get_all_position_is_dict(self):
+        """position within data is a dict with x, y, z."""
+        result = self.registry.get_all()
+
+        for entry in result:
+            pos = entry["data"]["position"]
+            assert isinstance(pos, dict)
+            assert "x" in pos
+            assert "y" in pos
+            assert "z" in pos
+
+    def test_get_all_ts_is_unix_timestamp(self):
+        """ts is an integer Unix timestamp."""
+        result = self.registry.get_all()
+
+        for entry in result:
+            assert isinstance(entry["ts"], int)
+            assert entry["ts"] > 0
+
+    @patch("infrastructure.visitor.time")
+    def test_get_all_uses_current_time(self, mock_time):
+        """ts is set from time.time()."""
+        mock_time.time.return_value = 1742529600
+
+        result = self.registry.get_all()
+        assert all(entry["ts"] == 1742529600 for entry in result)
+
+    def test_get_all_empty_registry(self):
+        """get_all() returns empty list when no visitors."""
+        self.registry.clear()
+        result = self.registry.get_all()
+        assert result == []
+
+
+# ---------------------------------------------------------------------------
+# VisitorRegistry.clear tests
+# ---------------------------------------------------------------------------
+
+
+class TestVisitorRegistryClear:
+    """Tests for VisitorRegistry.clear()."""
+
+    def setup_method(self):
+        """Clear registry and add test visitors."""
+        VisitorRegistry._instance = None
+        self.registry = VisitorRegistry()
+        self.registry.add("v1")
+        self.registry.add("v2")
+        self.registry.add("v3")
+
+    def teardown_method(self):
+        """Clean up after each test."""
+        VisitorRegistry._instance = None
+
+    def test_clear_removes_all_visitors(self):
+        """clear() removes all visitors from the registry."""
+        assert len(self.registry) == 3
+        self.registry.clear()
+        assert len(self.registry) == 0
+
+    def test_clear_allows_readding(self):
+        """Visitors can be re-added after clear()."""
+        self.registry.clear()
+        self.registry.add("v1")
+        assert len(self.registry) == 1