diff --git a/gateway/platforms/matrix.py b/gateway/platforms/matrix.py
index f7837a1f7d69..37d71becbd7e 100644
--- a/gateway/platforms/matrix.py
+++ b/gateway/platforms/matrix.py
@@ -485,11 +485,55 @@ def __init__(self, config: PlatformConfig):
         }
         self._approval_prompts_by_event: Dict[str, _MatrixApprovalPrompt] = {}
         self._approval_prompt_by_session: Dict[str, str] = {}
+        self.gateway_runner = None
         allowed_users_raw = os.getenv("MATRIX_ALLOWED_USERS", "")
         self._allowed_user_ids: Set[str] = {
             u.strip() for u in allowed_users_raw.split(",") if u.strip()
         }
 
+    @staticmethod
+    def _env_flag_enabled(name: str) -> bool:
+        return os.getenv(name, "").strip().lower() in {"true", "1", "yes"}
+
+    @staticmethod
+    def _env_allowlist(name: str) -> Set[str]:
+        raw = os.getenv(name, "").strip()
+        return {item.strip() for item in raw.split(",") if item.strip()}
+
+    def _is_reaction_approval_authorized(self, sender: str, room_id: str) -> bool:
+        """Return True when a Matrix reaction sender may resolve approvals."""
+        if not sender:
+            return False
+
+        auth_fn = getattr(getattr(self, "gateway_runner", None), "_is_user_authorized", None)
+        if callable(auth_fn):
+            try:
+                source = self.build_source(
+                    chat_id=room_id,
+                    chat_type="group",
+                    user_id=sender,
+                )
+                return bool(auth_fn(source))
+            except Exception as exc:
+                logger.warning(
+                    "Matrix: approval reaction auth fell back after runner check failed: %s",
+                    exc,
+                )
+
+        if self._env_flag_enabled("MATRIX_ALLOW_ALL_USERS"):
+            return True
+
+        matrix_allowed = set(self._allowed_user_ids) or self._env_allowlist(
+            "MATRIX_ALLOWED_USERS"
+        )
+        global_allowed = self._env_allowlist("GATEWAY_ALLOWED_USERS")
+
+        if not matrix_allowed and not global_allowed:
+            return self._env_flag_enabled("GATEWAY_ALLOW_ALL_USERS")
+
+        allowed = matrix_allowed | global_allowed
+        return "*" in allowed or sender in allowed
+
     def _is_duplicate_event(self, event_id) -> bool:
         """Return True if this event was already processed. Tracks the ID otherwise."""
         if not event_id:
@@ -2236,7 +2280,7 @@ async def _on_reaction(self, event: Any) -> None:
             if prompt and not prompt.resolved:
                 if room_id != prompt.chat_id:
                     return
-                if self._allowed_user_ids and sender not in self._allowed_user_ids:
+                if not self._is_reaction_approval_authorized(sender, room_id):
                     logger.info(
                         "Matrix: ignoring approval reaction from unauthorized user %s on %s",
                         sender, reacts_to,
diff --git a/gateway/run.py b/gateway/run.py
index 3e23cf2352af..f0a71ecd2120 100644
--- a/gateway/run.py
+++ b/gateway/run.py
@@ -6338,7 +6338,9 @@ def _create_adapter(
             if not check_matrix_requirements():
                 logger.warning("Matrix: mautrix not installed or credentials not set. Run: pip install 'mautrix[encryption]'")
                 return None
-            return MatrixAdapter(config)
+            adapter = MatrixAdapter(config)
+            adapter.gateway_runner = self
+            return adapter
 
         elif platform == Platform.API_SERVER:
             from gateway.platforms.api_server import APIServerAdapter, check_api_server_requirements
diff --git a/tests/gateway/test_matrix_exec_approval.py b/tests/gateway/test_matrix_exec_approval.py
index a7afe912cba5..6e7f481127bc 100644
--- a/tests/gateway/test_matrix_exec_approval.py
+++ b/tests/gateway/test_matrix_exec_approval.py
@@ -6,6 +6,45 @@
 from gateway.config import PlatformConfig
 
 
+def _clear_matrix_auth_env(monkeypatch):
+    for name in (
+        "MATRIX_ALLOWED_USERS",
+        "GATEWAY_ALLOWED_USERS",
+        "MATRIX_ALLOW_ALL_USERS",
+        "GATEWAY_ALLOW_ALL_USERS",
+    ):
+        monkeypatch.delenv(name, raising=False)
+
+
+def _make_adapter_with_prompt(monkeypatch):
+    from gateway.platforms.matrix import MatrixAdapter, _MatrixApprovalPrompt
+
+    adapter = MatrixAdapter(
+        PlatformConfig(
+            enabled=True,
+            token="tok",
+            extra={"homeserver": "https://matrix.example.org"},
+        )
+    )
+    # Resolve user_id so _is_self_sender doesn't defensively drop all traffic (#15763).
+    adapter._user_id = "@bot:example.org"
+    adapter._approval_prompts_by_event["$target"] = _MatrixApprovalPrompt(
+        session_key="sess-1", chat_id="!room:example.org", message_id="$target"
+    )
+    adapter._approval_prompt_by_session["sess-1"] = "$target"
+    return adapter
+
+
+def _reaction_event(sender):
+    content = {"m.relates_to": {"event_id": "$target", "key": "✅"}}
+    return types.SimpleNamespace(
+        sender=sender,
+        event_id=f"$react-{sender}",
+        room_id="!room:example.org",
+        content=content,
+    )
+
+
 class TestMatrixExecApprovalReactions:
     @pytest.mark.asyncio
     async def test_send_exec_approval_registers_prompt_and_seeds_reactions(self, monkeypatch):
@@ -58,3 +97,41 @@ async def test_reaction_resolves_pending_approval(self, monkeypatch):
         mock_resolve.assert_called_once_with("sess-1", "once")
         assert "$target" not in adapter._approval_prompts_by_event
         assert "sess-1" not in adapter._approval_prompt_by_session
+
+    @pytest.mark.asyncio
+    async def test_reaction_denied_when_matrix_allowlist_empty_and_global_mismatch(self, monkeypatch):
+        _clear_matrix_auth_env(monkeypatch)
+        monkeypatch.setenv("GATEWAY_ALLOWED_USERS", "@owner:example.org")
+        adapter = _make_adapter_with_prompt(monkeypatch)
+
+        with patch("tools.approval.resolve_gateway_approval", return_value=1) as mock_resolve:
+            await adapter._on_reaction(_reaction_event("@attacker:example.org"))
+
+        mock_resolve.assert_not_called()
+        assert "$target" in adapter._approval_prompts_by_event
+        assert adapter._approval_prompt_by_session["sess-1"] == "$target"
+
+    @pytest.mark.asyncio
+    async def test_reaction_allowed_by_global_allowlist_when_matrix_allowlist_empty(self, monkeypatch):
+        _clear_matrix_auth_env(monkeypatch)
+        monkeypatch.setenv("GATEWAY_ALLOWED_USERS", "@owner:example.org")
+        adapter = _make_adapter_with_prompt(monkeypatch)
+
+        with patch("tools.approval.resolve_gateway_approval", return_value=1) as mock_resolve:
+            await adapter._on_reaction(_reaction_event("@owner:example.org"))
+
+        mock_resolve.assert_called_once_with("sess-1", "once")
+        assert "$target" not in adapter._approval_prompts_by_event
+        assert "sess-1" not in adapter._approval_prompt_by_session
+
+    @pytest.mark.asyncio
+    async def test_reaction_denied_when_no_allowlist_is_configured(self, monkeypatch):
+        _clear_matrix_auth_env(monkeypatch)
+        adapter = _make_adapter_with_prompt(monkeypatch)
+
+        with patch("tools.approval.resolve_gateway_approval", return_value=1) as mock_resolve:
+            await adapter._on_reaction(_reaction_event("@attacker:example.org"))
+
+        mock_resolve.assert_not_called()
+        assert "$target" in adapter._approval_prompts_by_event
+        assert adapter._approval_prompt_by_session["sess-1"] == "$target"