diff --git a/packages/start-plugin-core/src/load-env-plugin/plugin.ts b/packages/start-plugin-core/src/load-env-plugin/plugin.ts
index d33b78d38bd..aecf837baad 100644
--- a/packages/start-plugin-core/src/load-env-plugin/plugin.ts
+++ b/packages/start-plugin-core/src/load-env-plugin/plugin.ts
@@ -5,8 +5,23 @@ export function loadEnvPlugin(): Plugin {
   return {
     name: 'tanstack-start-core:load-env',
     enforce: 'pre',
-    configResolved(config) {
-      Object.assign(process.env, loadEnv(config.mode, config.root, ''))
+    config(userConfig, { mode }) {
+      const root = userConfig.root || process.cwd()
+      const env = loadEnv(mode, root, '')
+
+      Object.assign(process.env, env)
+
+      const define: Record<string, string> = {}
+      for (const [key, value] of Object.entries(env)) {
+        define[`process.env.${key}`] = JSON.stringify(value)
+      }
+
+      return {
+        define: {
+          ...userConfig.define,
+          ...define,
+        },
+      }
     },
   }
 }
diff --git a/packages/start-plugin-core/tests/load-env/load-env.test.ts b/packages/start-plugin-core/tests/load-env/load-env.test.ts
new file mode 100644
index 00000000000..39231726c0c
--- /dev/null
+++ b/packages/start-plugin-core/tests/load-env/load-env.test.ts
@@ -0,0 +1,128 @@
+import { mkdirSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterEach, beforeEach, describe, expect, test } from 'vitest'
+
+import { loadEnvPlugin } from '../../src/load-env-plugin/plugin'
+
+describe('loadEnvPlugin', () => {
+  let testDir: string
+  const originalEnv = { ...process.env }
+
+  beforeEach(() => {
+    testDir = join(tmpdir(), `load-env-test-${Date.now()}`)
+    mkdirSync(testDir, { recursive: true })
+  })
+
+  afterEach(() => {
+    rmSync(testDir, { recursive: true, force: true })
+    for (const key of Object.keys(process.env)) {
+      if (!(key in originalEnv)) {
+        delete process.env[key]
+      }
+    }
+    Object.assign(process.env, originalEnv)
+  })
+
+  test('should load .env file', () => {
+    writeFileSync(join(testDir, '.env'), 'TEST_BASE_VAR=base_value')
+
+    const plugin = loadEnvPlugin()
+    const configHook = (plugin as any).config
+
+    const result = configHook({ root: testDir }, { mode: 'development' })
+
+    expect(process.env.TEST_BASE_VAR).toBe('base_value')
+    expect(result.define['process.env.TEST_BASE_VAR']).toBe('"base_value"')
+  })
+
+  test('should merge .env.development with .env', () => {
+    writeFileSync(
+      join(testDir, '.env'),
+      'TEST_SECRET=from_env\nTEST_SHARED=shared_value',
+    )
+    writeFileSync(
+      join(testDir, '.env.development'),
+      'TEST_SECRET=from_development\nTEST_DEV_ONLY=dev_value',
+    )
+
+    const plugin = loadEnvPlugin()
+    const configHook = (plugin as any).config
+
+    configHook({ root: testDir }, { mode: 'development' })
+
+    expect(process.env.TEST_SECRET).toBe('from_development')
+    expect(process.env.TEST_SHARED).toBe('shared_value')
+    expect(process.env.TEST_DEV_ONLY).toBe('dev_value')
+  })
+
+  test('should load .env.production in production mode', () => {
+    writeFileSync(join(testDir, '.env'), 'TEST_MODE_VAR=base')
+    writeFileSync(join(testDir, '.env.production'), 'TEST_MODE_VAR=production')
+    writeFileSync(
+      join(testDir, '.env.development'),
+      'TEST_MODE_VAR=development',
+    )
+
+    const plugin = loadEnvPlugin()
+    const configHook = (plugin as any).config
+
+    configHook({ root: testDir }, { mode: 'production' })
+
+    expect(process.env.TEST_MODE_VAR).toBe('production')
+  })
+
+  test('should load .env.development in development mode', () => {
+    writeFileSync(join(testDir, '.env'), 'TEST_MODE_VAR2=base')
+    writeFileSync(join(testDir, '.env.production'), 'TEST_MODE_VAR2=production')
+    writeFileSync(
+      join(testDir, '.env.development'),
+      'TEST_MODE_VAR2=development',
+    )
+
+    const plugin = loadEnvPlugin()
+    const configHook = (plugin as any).config
+
+    configHook({ root: testDir }, { mode: 'development' })
+
+    expect(process.env.TEST_MODE_VAR2).toBe('development')
+  })
+
+  test('should return define config with process.env replacements', () => {
+    writeFileSync(join(testDir, '.env'), 'TEST_API_KEY=secret123')
+
+    const plugin = loadEnvPlugin()
+    const configHook = (plugin as any).config
+
+    const result = configHook({ root: testDir }, { mode: 'development' })
+
+    expect(result.define['process.env.TEST_API_KEY']).toBe('"secret123"')
+  })
+
+  test('should preserve existing userConfig.define', () => {
+    writeFileSync(join(testDir, '.env'), 'TEST_VAR=value')
+
+    const plugin = loadEnvPlugin()
+    const configHook = (plugin as any).config
+
+    const result = configHook(
+      {
+        root: testDir,
+        define: { 'process.env.EXISTING': '"existing_value"' },
+      },
+      { mode: 'development' },
+    )
+
+    expect(result.define['process.env.EXISTING']).toBe('"existing_value"')
+    expect(result.define['process.env.TEST_VAR']).toBe('"value"')
+  })
+
+  test('should use process.cwd() when root is not specified', () => {
+    const plugin = loadEnvPlugin()
+    const configHook = (plugin as any).config
+
+    const result = configHook({}, { mode: 'development' })
+
+    expect(result).toHaveProperty('define')
+  })
+})