diff --git a/.changeset/fix-auth-js-absolute-basepath.md b/.changeset/fix-auth-js-absolute-basepath.md
new file mode 100644
index 000000000..6adc3b6b9
--- /dev/null
+++ b/.changeset/fix-auth-js-absolute-basepath.md
@@ -0,0 +1,5 @@
+---
+'@hono/auth-js': patch
+---
+
+Handle absolute URL in basePath of AuthConfigManager.setConfig()
diff --git a/packages/auth-js/src/client.test.ts b/packages/auth-js/src/client.test.ts
new file mode 100644
index 000000000..a59e53803
--- /dev/null
+++ b/packages/auth-js/src/client.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect } from 'vitest'
+import { normalizeBasePath } from './client'
+
+describe('normalizeBasePath', () => {
+  it('should split absolute http URL into baseUrl and basePath', () => {
+    const result = normalizeBasePath({ baseUrl: '', basePath: 'http://localhost:8000/api/auth' })
+    expect(result.baseUrl).toBe('http://localhost:8000')
+    expect(result.basePath).toBe('/api/auth')
+    expect(`${result.baseUrl}${result.basePath}/session`).toBe(
+      'http://localhost:8000/api/auth/session'
+    )
+  })
+
+  it('should split absolute https URL into baseUrl and basePath', () => {
+    const result = normalizeBasePath({ baseUrl: '', basePath: 'https://example.com/auth' })
+    expect(result.baseUrl).toBe('https://example.com')
+    expect(result.basePath).toBe('/auth')
+    expect(`${result.baseUrl}${result.basePath}/session`).toBe('https://example.com/auth/session')
+  })
+
+  it('should not modify relative basePath', () => {
+    const result = normalizeBasePath({ baseUrl: 'http://localhost:3000', basePath: '/custom/auth' })
+    expect(result.baseUrl).toBe('http://localhost:3000')
+    expect(result.basePath).toBe('/custom/auth')
+    expect(`${result.baseUrl}${result.basePath}/session`).toBe(
+      'http://localhost:3000/custom/auth/session'
+    )
+  })
+
+  it('should handle absolute URL without path', () => {
+    const result = normalizeBasePath({ baseUrl: '', basePath: 'http://localhost:8000' })
+    expect(result.baseUrl).toBe('http://localhost:8000')
+    expect(result.basePath).toBe('')
+    expect(`${result.baseUrl}${result.basePath}/session`).toBe('http://localhost:8000/session')
+  })
+
+  it('should override baseUrl when both are absolute URLs', () => {
+    const result = normalizeBasePath({
+      baseUrl: 'http://localhost:3000',
+      basePath: 'http://localhost:8000/api/auth',
+    })
+    expect(result.baseUrl).toBe('http://localhost:8000')
+    expect(result.basePath).toBe('/api/auth')
+    expect(`${result.baseUrl}${result.basePath}/session`).toBe(
+      'http://localhost:8000/api/auth/session'
+    )
+  })
+
+  it('should handle absolute URL with trailing slash', () => {
+    const result = normalizeBasePath({ baseUrl: '', basePath: 'http://localhost:8000/api/auth/' })
+    expect(result.baseUrl).toBe('http://localhost:8000')
+    expect(result.basePath).toBe('/api/auth')
+    expect(`${result.baseUrl}${result.basePath}/session`).toBe(
+      'http://localhost:8000/api/auth/session'
+    )
+  })
+})
diff --git a/packages/auth-js/src/client.ts b/packages/auth-js/src/client.ts
index 313f7df9e..48e25428c 100644
--- a/packages/auth-js/src/client.ts
+++ b/packages/auth-js/src/client.ts
@@ -173,6 +173,18 @@ interface ParsedUrl {
   toString: () => string
 }
 
+export function normalizeBasePath<T extends { basePath?: string }>(config: T): T {
+  if (config.basePath && /^https?:\/\//.test(config.basePath)) {
+    const url = new URL(config.basePath)
+    return {
+      ...config,
+      baseUrl: url.origin,
+      basePath: url.pathname.replace(/\/$/, ''),
+    }
+  }
+  return config
+}
+
 export function parseUrl(url?: string): ParsedUrl {
   const defaultUrl = 'http://localhost:3000/api/auth'
   const parsedUrl = new URL(url ? (url.startsWith('http') ? url : `https://${url}`) : defaultUrl)
diff --git a/packages/auth-js/src/react.tsx b/packages/auth-js/src/react.tsx
index 0ee40f5eb..dacfa69da 100644
--- a/packages/auth-js/src/react.tsx
+++ b/packages/auth-js/src/react.tsx
@@ -2,7 +2,14 @@ import type { BuiltInProviderType, RedirectableProviderType } from '@auth/core/p
 import type { LoggerInstance, Session } from '@auth/core/types'
 import * as React from 'react'
 import { useCallback, useContext, useEffect, useMemo, useState } from 'react'
-import { ClientSessionError, fetchData, now, parseUrl, useOnline } from './client'
+import {
+  ClientSessionError,
+  fetchData,
+  normalizeBasePath,
+  now,
+  parseUrl,
+  useOnline,
+} from './client'
 import type {
   WindowProps,
   AuthState,
@@ -53,7 +60,8 @@ class AuthConfigManager {
   }
 
   setConfig(userConfig: Partial<AuthClientConfig>): void {
-    this.config = { ...this.config, ...userConfig }
+    const normalized = normalizeBasePath(userConfig)
+    this.config = { ...this.config, ...normalized }
   }
 
   getConfig(): AuthClientConfig {