diff --git a/INSTALLATION.md b/INSTALLATION.md
index 3b3bc079..9b4ed879 100644
--- a/INSTALLATION.md
+++ b/INSTALLATION.md
@@ -69,6 +69,8 @@ Write this down. You'll use it in the callback URL below and again in step 4 whe
      - Pull requests: Read & write
      - Issues: Read & write
      - Metadata: Read-only
+   - **Organization permissions** (required only if you plan to set `ALLOWED_GITHUB_ORGS` — see step 5 / Security):
+     - Members: Read-only — used to verify org membership for the dashboard-login gate via `GET /orgs/{org}/memberships/{username}`. Without this permission that call returns 403, the check fails closed, and **every** dashboard login is rejected.
 4. Under **Subscribe to events**, enable:
    - `Issue comment`
    - `Pull request review`
@@ -225,6 +227,10 @@ ALLOWED_GITHUB_REPOS="some-user/their-repo,another-org/specific-repo"
 
 A GitHub or Linear webhook is accepted if the resolved repo's org is in `ALLOWED_GITHUB_ORGS` **or** the `owner/repo` is in `ALLOWED_GITHUB_REPOS`. If both are empty, all repos are allowed. Slack mentions are not rejected from regex-inferred repository text; repository access is bounded by the GitHub App installation permissions.
 
+`ALLOWED_GITHUB_ORGS` also gates **dashboard login**: when set, only GitHub accounts that are active members of one of the listed organizations can complete the OAuth login and receive a session. Membership is verified server-side with the GitHub App installation token (so private memberships are visible and no extra OAuth scope is required), and the check fails closed on any API error. When `ALLOWED_GITHUB_ORGS` is empty, dashboard login is open to any GitHub account (the prior behavior).
+
+> **Required GitHub App permission**: the membership check calls `GET /orgs/{org}/memberships/{username}`, which requires the GitHub App's **Organization → Members: Read-only** permission (see step 3b). If you set `ALLOWED_GITHUB_ORGS` without granting that permission, the call returns 403, the check fails closed, and **every** dashboard login is rejected. After changing an installed app's permissions, GitHub requires you to **approve the new permission** on each installation before it takes effect.
+
 ### Linear (optional)
 
 Open SWE listens for Linear comments that mention `@openswe`.
@@ -385,6 +391,8 @@ GITHUB_OAUTH_PROVIDER_ID=""            # The provider ID from steps 3a / 4b
 
 # === Repo Allowlist (optional) ===
 # Comma-separated list of GitHub orgs the agent is allowed to operate on.
+# Also gates dashboard login to members of these orgs (requires the GitHub App's
+# Organization -> Members: Read-only permission; without it, all dashboard logins are rejected).
 # Leave empty to allow all orgs.
 ALLOWED_GITHUB_ORGS=""                 # e.g. "my-org,my-other-org"
 # Comma-separated list of specific owner/repo pairs the agent is allowed to operate on.
diff --git a/agent/dashboard/oauth.py b/agent/dashboard/oauth.py
index 97cea7a3..7fc370cd 100644
--- a/agent/dashboard/oauth.py
+++ b/agent/dashboard/oauth.py
@@ -16,6 +16,8 @@ import httpx
 import jwt
 from fastapi import HTTPException, Request
 
+from agent.utils.github_org_membership import is_user_active_org_member
+
 logger = logging.getLogger(__name__)
 
 COOKIE_NAME = "osw_session"
@@ -80,6 +82,37 @@ def sanitize_redirect_to(redirect_to: str | None) -> str:
     return fallback
 
 
+def _allowed_login_orgs() -> frozenset[str]:
+    """Orgs whose members may log in to the dashboard.
+
+    Reuses the webhook-side ``ALLOWED_GITHUB_ORGS`` allowlist so deployments
+    configure a single org gate. When empty the dashboard login gate is
+    disabled (fail-open) to preserve existing deployments.
+    """
+    return frozenset(
+        org.strip().lower()
+        for org in os.environ.get("ALLOWED_GITHUB_ORGS", "").split(",")
+        if org.strip()
+    )
+
+
+async def enforce_org_login_gate(login: str) -> None:
+    """Reject dashboard login for users outside the allowed GitHub org(s).
+
+    No-op when ``ALLOWED_GITHUB_ORGS`` is unset. Otherwise the user must be an
+    active member of at least one configured org; membership is checked with
+    the GitHub App installation token (fail-closed on any API error).
+    """
+    orgs = _allowed_login_orgs()
+    if not orgs:
+        return
+    for org in orgs:
+        if await is_user_active_org_member(login, org):
+            return
+    logger.warning("Rejected dashboard login for %r — not in allowed org(s)", login)
+    raise HTTPException(403, "your GitHub account is not a member of an authorized organization")
+
+
 def issue_session(*, login: str, email: str | None, avatar_url: str | None) -> str:
     now = int(time.time())
     payload = {
diff --git a/agent/dashboard/routes.py b/agent/dashboard/routes.py
index 899dcb73..6f06df2e 100644
--- a/agent/dashboard/routes.py
+++ b/agent/dashboard/routes.py
@@ -24,6 +24,7 @@ from .oauth import (
     STATE_COOKIE_NAME,
     STATE_TTL_SECONDS,
     decode_state,
+    enforce_org_login_gate,
     exchange_code,
     fetch_github_user,
     hash_state_nonce,
@@ -188,6 +189,8 @@ async def auth_callback(request: Request, code: str, state: str) -> RedirectResp
     if not login:
         raise HTTPException(400, "could not resolve GitHub login")
 
+    await enforce_org_login_gate(login)
+
     await upsert_access_token_from_github_response(login, email or "", token_data)
 
     session_jwt = issue_session(login=login, email=email, avatar_url=user.get("avatar_url"))
diff --git a/agent/utils/github_org_membership.py b/agent/utils/github_org_membership.py
index 42ad8fee..5c972264 100644
--- a/agent/utils/github_org_membership.py
+++ b/agent/utils/github_org_membership.py
@@ -20,6 +20,10 @@ async def is_user_active_org_member(username: str, org: str) -> bool:
     memberships are visible (the same approach as the reference
     ``tag-external-contributions.yml`` workflow). On any API error, returns
     ``False`` — fail-closed for security.
+
+    Requires the GitHub App to have the ``Organization -> Members: Read-only``
+    permission; the ``GET /orgs/{org}/memberships/{username}`` endpoint returns
+    403 (-> ``False``) without it. See INSTALLATION.md.
     """
     if not username or not org:
         return False
diff --git a/tests/test_dashboard_org_login_gate.py b/tests/test_dashboard_org_login_gate.py
new file mode 100644
index 00000000..4283e466
--- /dev/null
+++ b/tests/test_dashboard_org_login_gate.py
@@ -0,0 +1,79 @@
+"""Tests for the dashboard GitHub-org login gate (ALLOWED_GITHUB_ORGS)."""
+
+from __future__ import annotations
+
+import pytest
+from fastapi import HTTPException
+
+from agent.dashboard import oauth
+
+
+def _stub_membership(monkeypatch, members: dict[str, set[str]]) -> dict[str, list[tuple[str, str]]]:
+    """Stub is_user_active_org_member; ``members`` maps org -> set of logins."""
+    seen: dict[str, list[tuple[str, str]]] = {"calls": []}
+
+    async def fake_is_user_active_org_member(username: str, org: str) -> bool:
+        seen["calls"].append((username, org))
+        return username in members.get(org, set())
+
+    monkeypatch.setattr(oauth, "is_user_active_org_member", fake_is_user_active_org_member)
+    return seen
+
+
+@pytest.mark.asyncio
+async def test_gate_noop_when_unset(monkeypatch) -> None:
+    monkeypatch.delenv("ALLOWED_GITHUB_ORGS", raising=False)
+    seen = _stub_membership(monkeypatch, {})
+
+    await oauth.enforce_org_login_gate("anyone")
+
+    assert seen["calls"] == []
+
+
+@pytest.mark.asyncio
+async def test_gate_noop_when_blank(monkeypatch) -> None:
+    monkeypatch.setenv("ALLOWED_GITHUB_ORGS", "  ,  ")
+    seen = _stub_membership(monkeypatch, {})
+
+    await oauth.enforce_org_login_gate("anyone")
+
+    assert seen["calls"] == []
+
+
+@pytest.mark.asyncio
+async def test_gate_allows_member(monkeypatch) -> None:
+    monkeypatch.setenv("ALLOWED_GITHUB_ORGS", "langchain-ai")
+    _stub_membership(monkeypatch, {"langchain-ai": {"insider"}})
+
+    await oauth.enforce_org_login_gate("insider")
+
+
+@pytest.mark.asyncio
+async def test_gate_rejects_non_member(monkeypatch) -> None:
+    monkeypatch.setenv("ALLOWED_GITHUB_ORGS", "langchain-ai")
+    _stub_membership(monkeypatch, {"langchain-ai": {"insider"}})
+
+    with pytest.raises(HTTPException) as exc:
+        await oauth.enforce_org_login_gate("stranger")
+
+    assert exc.value.status_code == 403
+
+
+@pytest.mark.asyncio
+async def test_gate_allows_member_of_any_configured_org(monkeypatch) -> None:
+    monkeypatch.setenv("ALLOWED_GITHUB_ORGS", "langchain-ai, anthropics")
+    _stub_membership(monkeypatch, {"anthropics": {"insider"}})
+
+    await oauth.enforce_org_login_gate("insider")
+
+
+@pytest.mark.asyncio
+async def test_gate_rejects_when_member_of_no_configured_org(monkeypatch) -> None:
+    monkeypatch.setenv("ALLOWED_GITHUB_ORGS", "langchain-ai,anthropics")
+    seen = _stub_membership(monkeypatch, {"other-org": {"stranger"}})
+
+    with pytest.raises(HTTPException) as exc:
+        await oauth.enforce_org_login_gate("stranger")
+
+    assert exc.value.status_code == 403
+    assert {org for _, org in seen["calls"]} == {"langchain-ai", "anthropics"}