diff --git a/cli/server.go b/cli/server.go
index dfd2db1dca9d8..1002966e1fd23 100644
--- a/cli/server.go
+++ b/cli/server.go
@@ -1006,7 +1006,7 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 			}
 			options.WebPushDispatcher = webpusher
 
-			githubOAuth2ConfigParams, err := getGithubOAuth2ConfigParams(ctx, options.Database, vals)
+			githubOAuth2ConfigParams, err := getGithubOAuth2ConfigParams(ctx, options.Logger, options.Database, vals)
 			if err != nil {
 				return xerrors.Errorf("get github oauth2 config params: %w", err)
 			}
@@ -2213,7 +2213,7 @@ func maybeAppendDefaultGithubExternalAuthProvider(
 	}), nil
 }
 
-func getGithubOAuth2ConfigParams(ctx context.Context, db database.Store, vals *codersdk.DeploymentValues) (*githubOAuth2ConfigParams, error) {
+func getGithubOAuth2ConfigParams(ctx context.Context, logger slog.Logger, db database.Store, vals *codersdk.DeploymentValues) (*githubOAuth2ConfigParams, error) {
 	params := githubOAuth2ConfigParams{
 		accessURL:         vals.AccessURL.Value(),
 		clientID:          vals.OAuth2.Github.ClientID.String(),
@@ -2250,6 +2250,16 @@ func getGithubOAuth2ConfigParams(ctx context.Context, db database.Store, vals *c
 	params.deviceFlow = GithubOAuth2DefaultProviderDeviceFlow
 	if len(params.allowOrgs) == 0 {
 		params.allowEveryone = GithubOAuth2DefaultProviderAllowEveryone
+	} else {
+		// The default provider is a GitHub App, which can only see memberships
+		// in organizations that have installed it. If the app isn't installed
+		// in an allowed organization, every login from that organization is
+		// rejected as "not a member".
+		logger.Warn(ctx, "the default GitHub OAuth provider can only see memberships in organizations that have installed the Coder GitHub app; "+
+			"users cannot log in until the app is installed in each allowed organization, or a custom GitHub OAuth app is configured",
+			slog.F("allowed_orgs", params.allowOrgs),
+			slog.F("install_url", coderd.GithubOAuth2DefaultProviderInstallURL),
+		)
 	}
 
 	return &params, nil
diff --git a/coderd/userauth.go b/coderd/userauth.go
index 4babef1be8f24..24bb95b7445d2 100644
--- a/coderd/userauth.go
+++ b/coderd/userauth.go
@@ -772,6 +772,20 @@ type GithubOAuth2Config struct {
 	DefaultProviderConfigured bool
 }
 
+const (
+	// GithubOAuth2DefaultProviderInstallURL is where admins install the
+	// default Coder GitHub app so it can see organization and team
+	// memberships.
+	GithubOAuth2DefaultProviderInstallURL = "https://github.com/apps/coder/installations/select_target"
+
+	// githubOAuth2DefaultProviderRemediation explains why the default GitHub
+	// app can fail org and team membership checks, and how to fix it. It is
+	// appended to login rejection messages and mirrored by the server startup
+	// warning and the GitHub auth docs, so keep those in sync.
+	githubOAuth2DefaultProviderRemediation = "The default GitHub OAuth provider can only see organizations and teams that have installed the Coder GitHub app. " +
+		"Install it from " + GithubOAuth2DefaultProviderInstallURL + " for each authorized organization, or configure a custom GitHub OAuth app."
+)
+
 func (*GithubOAuth2Config) PKCESupported() []promoauth.Oauth2PKCEChallengeMethod {
 	return []promoauth.Oauth2PKCEChallengeMethod{promoauth.PKCEChallengeMethodSha256}
 }
@@ -931,6 +945,13 @@ func (api *API) userOAuth2Github(rw http.ResponseWriter, r *http.Request) {
 		if len(selectedMemberships) == 0 {
 			status := http.StatusUnauthorized
 			msg := "You aren't a member of the authorized Github organizations!"
+			if api.GithubOAuth2Config.DefaultProviderConfigured {
+				// The default provider is a GitHub App, so it can only report
+				// memberships in organizations that have installed it. Without
+				// this hint, users in an allowed organization see a confusing
+				// rejection with no way to discover the missing installation.
+				msg += " " + githubOAuth2DefaultProviderRemediation
+			}
 			if api.GithubOAuth2Config.DeviceFlowEnabled {
 				// In the device flow, the error is rendered client-side.
 				httpapi.Write(ctx, rw, status, codersdk.Response{
@@ -977,6 +998,12 @@ func (api *API) userOAuth2Github(rw http.ResponseWriter, r *http.Request) {
 		}
 		if allowedTeam == nil {
 			msg := fmt.Sprintf("You aren't a member of an authorized team in the %v Github organization(s)!", organizationNames)
+			if api.GithubOAuth2Config.DefaultProviderConfigured {
+				// Team visibility has the same limitation as org visibility:
+				// the default GitHub App cannot see teams in organizations
+				// where it isn't installed.
+				msg += " " + githubOAuth2DefaultProviderRemediation
+			}
 			status := http.StatusUnauthorized
 			if api.GithubOAuth2Config.DeviceFlowEnabled {
 				// In the device flow, the error is rendered client-side.
diff --git a/coderd/userauth_test.go b/coderd/userauth_test.go
index 463ce8365128b..bba1f93e1072b 100644
--- a/coderd/userauth_test.go
+++ b/coderd/userauth_test.go
@@ -221,6 +221,63 @@ func TestUserOAuth2Github(t *testing.T) {
 
 		resp := oauth2Callback(t, client)
 		require.Equal(t, http.StatusUnauthorized, resp.StatusCode)
+		location, err := resp.Location()
+		require.NoError(t, err)
+		require.NotContains(t, location.Query().Get("message"), "Coder GitHub app")
+	})
+	t.Run("NotInAllowedOrganizationDefaultProvider", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, &coderdtest.Options{
+			GithubOAuth2Config: &coderd.GithubOAuth2Config{
+				OAuth2Config:              &testutil.OAuth2Config{},
+				DefaultProviderConfigured: true,
+				AllowOrganizations:        []string{"coder"},
+				ListOrganizationMemberships: func(ctx context.Context, client *http.Client) ([]*github.Membership, error) {
+					// The default provider is a GitHub App, so it reports no
+					// memberships for organizations it isn't installed in.
+					return []*github.Membership{}, nil
+				},
+			},
+		})
+
+		resp := oauth2Callback(t, client)
+		require.Equal(t, http.StatusUnauthorized, resp.StatusCode)
+		// The error must tell the user how to fix the likely cause: the Coder
+		// GitHub app isn't installed in the allowed organization.
+		location, err := resp.Location()
+		require.NoError(t, err)
+		require.Contains(t, location.Query().Get("message"), "Coder GitHub app")
+		require.Contains(t, location.Query().Get("message"), "https://github.com/apps/coder")
+	})
+	t.Run("NotInAllowedOrganizationDefaultProviderDeviceFlow", func(t *testing.T) {
+		t.Parallel()
+		client := coderdtest.New(t, &coderdtest.Options{
+			GithubOAuth2Config: &coderd.GithubOAuth2Config{
+				OAuth2Config:              &testutil.OAuth2Config{},
+				DefaultProviderConfigured: true,
+				AllowOrganizations:        []string{"coder"},
+				ListOrganizationMemberships: func(ctx context.Context, client *http.Client) ([]*github.Membership, error) {
+					return []*github.Membership{}, nil
+				},
+				DeviceFlowEnabled: true,
+				ExchangeDeviceCode: func(_ context.Context, _ string) (*oauth2.Token, error) {
+					return &oauth2.Token{
+						AccessToken:  "access_token",
+						RefreshToken: "refresh_token",
+						Expiry:       time.Now().Add(time.Hour),
+					}, nil
+				},
+			},
+		})
+
+		resp := oauth2Callback(t, client)
+		require.Equal(t, http.StatusUnauthorized, resp.StatusCode)
+		// In the device flow the error is rendered client-side, so the hint
+		// must arrive in the response body Detail rather than the redirect.
+		var apiErr codersdk.Response
+		require.NoError(t, json.NewDecoder(resp.Body).Decode(&apiErr))
+		require.Contains(t, apiErr.Detail, "Coder GitHub app")
+		require.Contains(t, apiErr.Detail, "https://github.com/apps/coder")
 	})
 	t.Run("NotInAllowedTeam", func(t *testing.T) {
 		t.Parallel()
diff --git a/docs/admin/users/github-auth.md b/docs/admin/users/github-auth.md
index 4d07abb1e2e18..9125c6217b441 100644
--- a/docs/admin/users/github-auth.md
+++ b/docs/admin/users/github-auth.md
@@ -44,6 +44,11 @@ To use the default configuration:
    CODER_OAUTH2_GITHUB_ALLOWED_ORGS="your-org"
    ```
 
+   > [!IMPORTANT]
+   > The default GitHub app can only see memberships in organizations where it is installed.
+   > If you set `CODER_OAUTH2_GITHUB_ALLOWED_ORGS` without installing the app in each allowed organization, all logins fail with "You aren't a member of the authorized Github organizations!", including the first admin login on a fresh deployment.
+   > Install the app for each organization at the [Coder app on GitHub](https://github.com/apps/coder/installations/select_target).
+
 ## Disable the Default GitHub App
 
 You can disable the default GitHub app by [configuring your own app](#step-1-configure-the-oauth-application-in-github)