diff --git a/__test__/git-auth-helper.test.ts b/__test__/git-auth-helper.test.ts
index 2c963b9..0e5f14a 100644
--- a/__test__/git-auth-helper.test.ts
+++ b/__test__/git-auth-helper.test.ts
@@ -923,6 +923,25 @@ describe('git-auth-helper tests', () => {
     }
   })
 
+  const configureGlobalAuth_overridesGitConfigGlobal =
+    'configureGlobalAuth overrides GIT_CONFIG_GLOBAL'
+  it(configureGlobalAuth_overridesGitConfigGlobal, async () => {
+    // Arrange
+    await setup(configureGlobalAuth_overridesGitConfigGlobal)
+    const authHelper = gitAuthHelper.createAuthHelper(git, settings)
+
+    // Act
+    await authHelper.configureAuth()
+    await authHelper.configureGlobalAuth()
+
+    // Assert GIT_CONFIG_GLOBAL is pinned to the temporary global config, so an
+    // inherited GIT_CONFIG_GLOBAL cannot redirect --global writes
+    expect(git.env['HOME']).toBeTruthy()
+    expect(git.env['GIT_CONFIG_GLOBAL']).toBe(
+      path.join(git.env['HOME'], '.gitconfig')
+    )
+  })
+
   const removeGlobalConfig_removesOverride =
     'removeGlobalConfig removes override'
   it(removeGlobalConfig_removesOverride, async () => {
@@ -933,6 +952,7 @@ describe('git-auth-helper tests', () => {
     await authHelper.configureGlobalAuth()
     const homeOverride = git.env['HOME'] // Sanity check
     expect(homeOverride).toBeTruthy()
+    expect(git.env['GIT_CONFIG_GLOBAL']).toBeTruthy()
     await fs.promises.stat(path.join(git.env['HOME'], '.gitconfig'))
 
     // Act
@@ -940,6 +960,7 @@ describe('git-auth-helper tests', () => {
 
     // Assert
     expect(git.env['HOME']).toBeUndefined()
+    expect(git.env['GIT_CONFIG_GLOBAL']).toBeUndefined()
     try {
       await fs.promises.stat(homeOverride)
       throw new Error(`Should have been deleted '${homeOverride}'`)
diff --git a/dist/index.js b/dist/index.js
index b381bd2..13faf11 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -35109,6 +35109,10 @@ class GitAuthHelper {
         // Override HOME
         info(`Temporarily overriding HOME='${this.temporaryHomePath}' before making global git config changes`);
         this.git.setEnvironmentVariable('HOME', this.temporaryHomePath);
+        // GIT_CONFIG_GLOBAL takes precedence over HOME when locating the global
+        // config file. Pin it to the temporary config so an inherited
+        // GIT_CONFIG_GLOBAL cannot redirect our global git config writes elsewhere.
+        this.git.setEnvironmentVariable('GIT_CONFIG_GLOBAL', newGitConfigPath);
         return newGitConfigPath;
     }
     async configureGlobalAuth() {
@@ -35183,8 +35187,9 @@ class GitAuthHelper {
     }
     async removeGlobalConfig() {
         if (this.temporaryHomePath?.length > 0) {
-            core_debug(`Unsetting HOME override`);
+            core_debug(`Unsetting HOME and GIT_CONFIG_GLOBAL overrides`);
             this.git.removeEnvironmentVariable('HOME');
+            this.git.removeEnvironmentVariable('GIT_CONFIG_GLOBAL');
             await rmRF(this.temporaryHomePath);
         }
     }
diff --git a/src/git-auth-helper.ts b/src/git-auth-helper.ts
index dd7e6fb..e2d1054 100644
--- a/src/git-auth-helper.ts
+++ b/src/git-auth-helper.ts
@@ -122,6 +122,11 @@ class GitAuthHelper {
     )
     this.git.setEnvironmentVariable('HOME', this.temporaryHomePath)
 
+    // GIT_CONFIG_GLOBAL takes precedence over HOME when locating the global
+    // config file. Pin it to the temporary config so an inherited
+    // GIT_CONFIG_GLOBAL cannot redirect our global git config writes elsewhere.
+    this.git.setEnvironmentVariable('GIT_CONFIG_GLOBAL', newGitConfigPath)
+
     return newGitConfigPath
   }
 
@@ -237,8 +242,9 @@ class GitAuthHelper {
 
   async removeGlobalConfig(): Promise<void> {
     if (this.temporaryHomePath?.length > 0) {
-      core.debug(`Unsetting HOME override`)
+      core.debug(`Unsetting HOME and GIT_CONFIG_GLOBAL overrides`)
       this.git.removeEnvironmentVariable('HOME')
+      this.git.removeEnvironmentVariable('GIT_CONFIG_GLOBAL')
       await io.rmRF(this.temporaryHomePath)
     }
   }