diff --git a/packages/oauth-provider/src/introspect.ts b/packages/oauth-provider/src/introspect.ts
index c4b41b2e44..7b29234527 100644
--- a/packages/oauth-provider/src/introspect.ts
+++ b/packages/oauth-provider/src/introspect.ts
@@ -17,6 +17,7 @@ import {
 	getClient,
 	getJwtPlugin,
 	getStoredToken,
+	parseClientMetadata,
 	validateClientCredentials,
 } from "./utils";
 
@@ -218,7 +219,7 @@ async function validateOpaqueAccessToken(
 				user,
 				scopes: accessToken.scopes,
 				referenceId: accessToken?.referenceId,
-				metadata: client?.metadata ? JSON.parse(client.metadata) : undefined,
+				metadata: parseClientMetadata(client?.metadata),
 			})
 		: {};
 
diff --git a/packages/oauth-provider/src/register.test.ts b/packages/oauth-provider/src/register.test.ts
index fc1c800d54..b6516d2947 100644
--- a/packages/oauth-provider/src/register.test.ts
+++ b/packages/oauth-provider/src/register.test.ts
@@ -210,6 +210,41 @@ describe("oauth register", async () => {
 
 		expect(response.data?.disabled).toBeFalsy();
 	});
+
+	it("should register client with metadata field", async () => {
+		const response = await auth.api.adminCreateOAuthClient({
+			headers,
+			body: {
+				redirect_uris: [redirectUri],
+				metadata: {
+					foo: "bar",
+					nested: { key: "value" },
+				},
+			},
+		});
+		expect(response?.client_id).toBeDefined();
+		expect(response?.client_secret).toBeDefined();
+		// Metadata should be spread at the top level of the response
+		expect(response?.foo).toBe("bar");
+		expect(response?.nested).toEqual({ key: "value" });
+	});
+
+	it("should register client with metadata and other extra fields", async () => {
+		const response = await auth.api.adminCreateOAuthClient({
+			headers,
+			body: {
+				redirect_uris: [redirectUri],
+				metadata: {
+					fromMetadata: "value1",
+				},
+				customField: "value2",
+			} as any,
+		});
+		expect(response?.client_id).toBeDefined();
+		// Both metadata and extra fields should be spread at the top level
+		expect(response?.fromMetadata).toBe("value1");
+		expect(response?.customField).toBe("value2");
+	});
 });
 
 describe("oauth register - unauthenticated", async () => {
diff --git a/packages/oauth-provider/src/register.ts b/packages/oauth-provider/src/register.ts
index 324df23839..5c49811301 100644
--- a/packages/oauth-provider/src/register.ts
+++ b/packages/oauth-provider/src/register.ts
@@ -4,7 +4,7 @@ import { generateRandomString } from "better-auth/crypto";
 import { toExpJWT } from "better-auth/plugins";
 import type { OAuthOptions, SchemaClient, Scope } from "./types";
 import type { OAuthClient } from "./types/oauth";
-import { storeClientSecret } from "./utils";
+import { parseClientMetadata, storeClientSecret } from "./utils";
 
 export async function registerEndpoint(
 	ctx: GenericEndpointContext,
@@ -252,6 +252,7 @@ export function oauthToSchema(input: OAuthClient): SchemaClient<Scope[]> {
 		skip_consent: skipConsent,
 		enable_end_session: enableEndSession,
 		reference_id: referenceId,
+		metadata: inputMetadata,
 		// All other metadata
 		...rest
 	} = input;
@@ -260,8 +261,15 @@ export function oauthToSchema(input: OAuthClient): SchemaClient<Scope[]> {
 	const expiresAt = _expiresAt ? new Date(_expiresAt * 1000) : undefined;
 	const createdAt = _createdAt ? new Date(_createdAt * 1000) : undefined;
 	const scopes = _scope?.split(" ");
-	const metadata =
-		rest && Object.keys(rest).length ? JSON.stringify(rest) : undefined;
+	const metadataObj = {
+		...(rest && Object.keys(rest).length ? rest : {}),
+		...(inputMetadata && typeof inputMetadata === "object"
+			? inputMetadata
+			: {}),
+	};
+	const metadata = Object.keys(metadataObj).length
+		? JSON.stringify(metadataObj)
+		: undefined;
 
 	return {
 		// Important Fields
@@ -354,7 +362,7 @@ export function schemaToOAuth(input: SchemaClient<Scope[]>): OAuthClient {
 		? Math.round(createdAt.getTime() / 1000)
 		: undefined;
 	const _scopes = scopes?.join(" ");
-	const _metadata = metadata ? JSON.parse(metadata) : undefined;
+	const _metadata = parseClientMetadata(metadata);
 
 	return {
 		// All other metadata
diff --git a/packages/oauth-provider/src/token.ts b/packages/oauth-provider/src/token.ts
index e8a64fc6f8..d1b99493d1 100644
--- a/packages/oauth-provider/src/token.ts
+++ b/packages/oauth-provider/src/token.ts
@@ -20,6 +20,7 @@ import {
 	decryptStoredClientSecret,
 	getJwtPlugin,
 	getStoredToken,
+	parseClientMetadata,
 	storeToken,
 	validateClientCredentials,
 } from "./utils";
@@ -85,7 +86,7 @@ async function createJwtAccessToken(
 				scopes,
 				resource: ctx.body.resource,
 				referenceId,
-				metadata: client.metadata ? JSON.parse(client.metadata) : undefined,
+				metadata: parseClientMetadata(client.metadata),
 			})
 		: {};
 
@@ -142,7 +143,7 @@ async function createIdToken(
 		? await opts.customIdTokenClaims({
 				user,
 				scopes,
-				metadata: client.metadata ? JSON.parse(client.metadata) : undefined,
+				metadata: parseClientMetadata(client.metadata),
 			})
 		: {};
 
@@ -817,7 +818,7 @@ async function handleClientCredentialsGrant(
 		? await opts.customAccessTokenClaims({
 				scopes: requestedScopes,
 				resource: ctx.body.resource,
-				metadata: client.metadata ? JSON.parse(client.metadata) : undefined,
+				metadata: parseClientMetadata(client.metadata),
 			})
 		: {};
 
diff --git a/packages/oauth-provider/src/utils/index.ts b/packages/oauth-provider/src/utils/index.ts
index 9f5e75ea26..a2e0f167a7 100644
--- a/packages/oauth-provider/src/utils/index.ts
+++ b/packages/oauth-provider/src/utils/index.ts
@@ -374,6 +374,19 @@ export async function validateClientCredentials(
 	return client;
 }
 
+/**
+ * Parse client metadata that may be stored as JSON string or already parsed object.
+ * Handles database adapters that auto-parse JSON columns.
+ *
+ * @internal
+ */
+export function parseClientMetadata(
+	metadata: string | object | undefined,
+): object | undefined {
+	if (!metadata) return undefined;
+	return typeof metadata === "string" ? JSON.parse(metadata) : metadata;
+}
+
 /**
  * Parse space-separated prompt string into a set of prompts
  *