diff --git a/internal/api/token_access_token.go b/internal/api/token_access_token.go
index 0f693dd53..38338ab44 100644
--- a/internal/api/token_access_token.go
+++ b/internal/api/token_access_token.go
@@ -15,8 +15,9 @@ import (
 
 // AccessTokenGrantParams are the parameters the AccessTokenGrant method accepts
 type AccessTokenGrantParams struct {
-	Provider    string `json:"provider"`
-	AccessToken string `json:"access_token"`
+	Provider     string `json:"provider"`
+	AccessToken  string `json:"access_token"`
+	LinkIdentity bool   `json:"link_identity"`
 }
 
 // AccessTokenGrant implements the access_token grant type flow, which allows
@@ -28,6 +29,10 @@ type AccessTokenGrantParams struct {
 // an OIDC id token (AuthenticationToken) on the first authorization, which
 // makes the id_token grant unusable for repeat logins without falling back to
 // the browser flow.
+//
+// When link_identity is set and a valid user access token is provided in the
+// Authorization header, the provider identity is linked to that user instead of
+// signing in or creating a new account.
 func (a *API) AccessTokenGrant(ctx context.Context, w http.ResponseWriter, r *http.Request) error {
 	db := a.db.WithContext(ctx)
 
@@ -44,6 +49,25 @@ func (a *API) AccessTokenGrant(ctx context.Context, w http.ResponseWriter, r *ht
 		return apierrors.NewOAuthError("invalid request", "provider required")
 	}
 
+	if params.LinkIdentity {
+		if r.Header.Get("Authorization") == "" {
+			return apierrors.NewOAuthError("invalid request", "Linking requires a valid user access token in Authorization")
+		}
+
+		requireAuthCtx, err := a.requireAuthentication(w, r)
+		if err != nil {
+			return err
+		}
+
+		targetUser := getUser(requireAuthCtx)
+		if targetUser == nil {
+			return apierrors.NewOAuthError("invalid request", "Linking requires a valid user authentication")
+		}
+
+		// set it so linkIdentityToUser works below
+		ctx = withTargetUser(ctx, targetUser)
+	}
+
 	oauthProvider, pConfig, err := a.OAuthProvider(ctx, params.Provider)
 	if err != nil {
 		return apierrors.NewBadRequestError(apierrors.ErrorCodeOAuthProviderNotSupported, "Unsupported provider: %q", params.Provider).WithInternalError(err)
@@ -84,8 +108,10 @@ func (a *API) AccessTokenGrant(ctx context.Context, w http.ResponseWriter, r *ht
 	var grantParams models.GrantParams
 	grantParams.FillGrantParams(r)
 
-	if err := a.triggerBeforeUserCreatedExternal(r, db, userData, params.Provider); err != nil {
-		return err
+	if !params.LinkIdentity {
+		if err := a.triggerBeforeUserCreatedExternal(r, db, userData, params.Provider); err != nil {
+			return err
+		}
 	}
 
 	var createdUser bool
@@ -95,7 +121,11 @@ func (a *API) AccessTokenGrant(ctx context.Context, w http.ResponseWriter, r *ht
 		var terr error
 
 		var decision models.AccountLinkingDecision
-		decision, user, terr = a.createAccountFromExternalIdentity(tx, r, userData, params.Provider, pConfig.EmailOptional)
+		if params.LinkIdentity {
+			user, terr = a.linkIdentityToUser(r, ctx, tx, userData, params.Provider)
+		} else {
+			decision, user, terr = a.createAccountFromExternalIdentity(tx, r, userData, params.Provider, pConfig.EmailOptional)
+		}
 		if terr != nil {
 			return terr
 		}
diff --git a/internal/api/token_access_token_test.go b/internal/api/token_access_token_test.go
index dd20c0f1f..620914d7f 100644
--- a/internal/api/token_access_token_test.go
+++ b/internal/api/token_access_token_test.go
@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+
+	"github.com/supabase/auth/internal/models"
 )
 
 // FacebookAccessTokenSetup spins up a mock Graph API that answers the
@@ -146,3 +148,65 @@ func (ts *ExternalTestSuite) TestAccessTokenGrantSignupDisabled() {
 	w := ts.accessTokenGrant("facebook", "valid_access_token")
 	ts.Require().Equal(http.StatusUnprocessableEntity, w.Code)
 }
+
+func (ts *ExternalTestSuite) generateAccessTokenAndSession(u *models.User) string {
+	s, err := models.NewSession(u.ID, nil)
+	ts.Require().NoError(err)
+	ts.Require().NoError(ts.API.db.Create(s))
+
+	req := httptest.NewRequest(http.MethodPost, "/token?grant_type=password", nil)
+	token, _, err := ts.API.generateAccessToken(req, ts.API.db, u, &s.ID, models.PasswordGrant)
+	ts.Require().NoError(err)
+	return token
+}
+
+func (ts *ExternalTestSuite) accessTokenLinkGrant(provider, accessToken, authToken string) *httptest.ResponseRecorder {
+	var buffer bytes.Buffer
+	ts.Require().NoError(json.NewEncoder(&buffer).Encode(map[string]interface{}{
+		"provider":      provider,
+		"access_token":  accessToken,
+		"link_identity": true,
+	}))
+
+	req := httptest.NewRequest(http.MethodPost, "http://localhost/token?grant_type=access_token", &buffer)
+	req.Header.Set("Content-Type", "application/json")
+	if authToken != "" {
+		req.Header.Set("Authorization", "Bearer "+authToken)
+	}
+	w := httptest.NewRecorder()
+	ts.API.handler.ServeHTTP(w, req)
+	return w
+}
+
+func (ts *ExternalTestSuite) TestAccessTokenGrantLinkIdentity() {
+	user, err := ts.createUser("link-user-sub", "linkme@example.com", "Link Me", "", "")
+	ts.Require().NoError(err)
+	token := ts.generateAccessTokenAndSession(user)
+
+	server := FacebookAccessTokenSetup(ts, ts.Config.External.Facebook.ClientID[0], true, "USER", facebookUser)
+	defer server.Close()
+
+	w := ts.accessTokenLinkGrant("facebook", "valid_access_token", token)
+	ts.Require().Equal(http.StatusOK, w.Code, w.Body.String())
+
+	var response AccessTokenResponse
+	ts.Require().NoError(json.NewDecoder(w.Body).Decode(&response))
+	ts.Require().NotNil(response.User)
+	// Linking attaches the facebook identity to the already-authenticated user.
+	ts.Equal(user.ID, response.User.ID)
+
+	ts.Require().NoError(ts.API.db.Load(user, "Identities"))
+	var providers []string
+	for _, identity := range user.Identities {
+		providers = append(providers, identity.Provider)
+	}
+	ts.Contains(providers, "facebook")
+}
+
+func (ts *ExternalTestSuite) TestAccessTokenGrantLinkIdentityRequiresAuth() {
+	server := FacebookAccessTokenSetup(ts, ts.Config.External.Facebook.ClientID[0], true, "USER", facebookUser)
+	defer server.Close()
+
+	w := ts.accessTokenLinkGrant("facebook", "valid_access_token", "")
+	ts.Require().Equal(http.StatusBadRequest, w.Code)
+}