diff --git a/routers/web/auth/oauth2_provider.go b/routers/web/auth/oauth2_provider.go
index edeb46e361e..c2b44bd6620 100644
--- a/routers/web/auth/oauth2_provider.go
+++ b/routers/web/auth/oauth2_provider.go
@@ -131,12 +131,21 @@ func InfoOAuth(ctx *context.Context) {
 	ctx.JSON(http.StatusOK, response)
 }
 
+// RFC 6749 section 2.3.1: client_secret_basic credentials are form-urlencoded
+func parseClientSecretBasic(authHeader string) (clientID, clientSecret string, ok bool) {
+	parsed, _ := httpauth.ParseAuthorizationHeader(authHeader)
+	if parsed.BasicAuth == nil {
+		return "", "", false
+	}
+	clientID, errID := url.QueryUnescape(parsed.BasicAuth.Username)
+	clientSecret, errSecret := url.QueryUnescape(parsed.BasicAuth.Password)
+	return clientID, clientSecret, errID == nil && errSecret == nil
+}
+
 // IntrospectOAuth introspects an oauth token
 func IntrospectOAuth(ctx *context.Context) {
 	var introspectingApp *auth.OAuth2Application
-	authHeader := ctx.Req.Header.Get("Authorization")
-	if parsed, ok := httpauth.ParseAuthorizationHeader(authHeader); ok && parsed.BasicAuth != nil {
-		clientID, clientSecret := parsed.BasicAuth.Username, parsed.BasicAuth.Password
+	if clientID, clientSecret, ok := parseClientSecretBasic(ctx.Req.Header.Get("Authorization")); ok {
 		app, err := auth.GetOAuth2ApplicationByClientID(ctx, clientID)
 		if err != nil && !auth.IsErrOauthClientIDInvalid(err) {
 			// this is likely a database error; log it and respond without details
@@ -499,15 +508,14 @@ func AccessTokenOAuth(ctx *context.Context) {
 	// if there is no ClientID or ClientSecret in the request body, fill these fields by the Authorization header and ensure the provided field matches the Authorization header
 	if form.ClientID == "" || form.ClientSecret == "" {
 		if authHeader := ctx.Req.Header.Get("Authorization"); authHeader != "" {
-			parsed, ok := httpauth.ParseAuthorizationHeader(authHeader)
-			if !ok || parsed.BasicAuth == nil {
+			clientID, clientSecret, ok := parseClientSecretBasic(authHeader)
+			if !ok {
 				handleAccessTokenError(ctx, oauth2_provider.AccessTokenError{
 					ErrorCode:        oauth2_provider.AccessTokenErrorCodeInvalidRequest,
 					ErrorDescription: "cannot parse basic auth header",
 				})
 				return
 			}
-			clientID, clientSecret := parsed.BasicAuth.Username, parsed.BasicAuth.Password
 			// validate that any fields present in the form match the Basic auth header
 			if form.ClientID != "" && form.ClientID != clientID {
 				handleAccessTokenError(ctx, oauth2_provider.AccessTokenError{
diff --git a/routers/web/auth/oauth2_provider_test.go b/routers/web/auth/oauth2_provider_test.go
new file mode 100644
index 00000000000..aa3a0ae898d
--- /dev/null
+++ b/routers/web/auth/oauth2_provider_test.go
@@ -0,0 +1,71 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"encoding/base64"
+	"net/http"
+	"strings"
+	"testing"
+
+	"gitea.dev/models/auth"
+	"gitea.dev/models/unittest"
+	"gitea.dev/modules/json"
+	"gitea.dev/modules/test"
+	"gitea.dev/modules/web"
+	"gitea.dev/services/contexttest"
+	"gitea.dev/services/forms"
+	"gitea.dev/services/oauth2_provider"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestOAuth2FormEncodedBasicAuth(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+	signingKey, err := oauth2_provider.CreateJWTSigningKey("HS256", make([]byte, 32))
+	require.NoError(t, err)
+	defer test.MockVariableValue(&oauth2_provider.DefaultSigningKey, signingKey)()
+
+	app := unittest.AssertExistsAndLoadBean(t, &auth.OAuth2Application{ID: 1})
+	const fixtureClientSecretPlaintext = "4MK8Na6R55smdCY0WuCCumZ6hjRPnGY5saWVRHHjJiA="
+	escape := strings.NewReplacer("-", "%2D", "=", "%3D").Replace
+
+	t.Run("AccessToken accepts client matching body client_id and rejects unknown code", func(t *testing.T) {
+		ctx, resp := contexttest.MockContext(t, "POST /login/oauth/access_token")
+		ctx.Req.SetBasicAuth(escape(app.ClientID), escape(fixtureClientSecretPlaintext))
+		web.SetForm(ctx, &forms.AccessTokenForm{GrantType: "authorization_code", ClientID: app.ClientID})
+		AccessTokenOAuth(ctx)
+
+		var accessTokenErr oauth2_provider.AccessTokenError
+		require.NoError(t, json.Unmarshal(resp.Body.Bytes(), &accessTokenErr))
+		assert.Equal(t, "client is not authorized", accessTokenErr.ErrorDescription)
+	})
+
+	t.Run("Introspect accepts client and reports inactive token", func(t *testing.T) {
+		ctx, resp := contexttest.MockContext(t, "POST /login/oauth/introspect")
+		ctx.Req.SetBasicAuth(escape(app.ClientID), escape(fixtureClientSecretPlaintext))
+		web.SetForm(ctx, &forms.IntrospectTokenForm{Token: "invalid"})
+		IntrospectOAuth(ctx)
+		assert.Equal(t, http.StatusOK, resp.Code)
+	})
+}
+
+func TestParseClientSecretBasic(t *testing.T) {
+	basicAuth := func(username, password string) string {
+		return "Basic " + base64.StdEncoding.EncodeToString([]byte(username+":"+password))
+	}
+
+	clientID, clientSecret, ok := parseClientSecretBasic(basicAuth("a+b", "c%2Bd"))
+	assert.True(t, ok)
+	assert.Equal(t, "a b", clientID)
+	assert.Equal(t, "c+d", clientSecret)
+
+	_, _, ok = parseClientSecretBasic(basicAuth("a%zz", "c"))
+	assert.False(t, ok)
+	_, _, ok = parseClientSecretBasic(basicAuth("a", "c%zz"))
+	assert.False(t, ok)
+	_, _, ok = parseClientSecretBasic("Bearer token")
+	assert.False(t, ok)
+}