How to Fix 'APIClient.dll Was Not Found' on Windows 10

If you see “APIClient.dll was not found” when opening Word, Excel, Outlook, or another Microsoft 365 app on Windows 10, start by repairing Microsoft 365. The file is associated with Microsoft Office Click-to-Run, so downloading a loose DLL from a third-party site is not the right fix. If repair does not restore the Click-to-Run files, a clean Microsoft 365 reinstall is the next step.

What the APIClient.dll error means

The message commonly points to a location such as C:\Program Files\Common Files\Microsoft Shared\ClickToRun\ApiClient.dll. Even when the file is present, the Microsoft 365 Click-to-Run installation can still be damaged or incomplete, so Windows may fail to load the component correctly.

For that reason, do not treat this as a normal “find the missing DLL” problem. Microsoft’s supported troubleshooting path for broken Office files is to repair the Office installation, then uninstall and reinstall it if necessary.

1. Repair Microsoft 365 first

This is the best first fix because it replaces damaged Office program files without requiring you to hunt for individual DLLs.

1. Close Word, Excel, Outlook, Teams, and other Microsoft 365 desktop apps.
2. Press Windows + I and open Apps > Apps & features in Windows 10.
3. Select your Microsoft 365 or Microsoft Office installation, then choose Modify.
4. Run Quick Repair first. Restart the affected app and check whether the error is gone.
5. If the error remains, return to the same repair window and choose Online Repair. This is the more thorough repair option and requires an internet connection.
6. Restart Windows after the repair completes, then open a Microsoft 365 app again.

Microsoft documents both repair modes in its Office repair instructions. Quick Repair is faster, while Online Repair is intended to perform a more complete repair.

2. Update Microsoft 365 if an Office app still opens

If at least one Office app opens normally after the repair, install any available Office updates before doing a full reinstall. In Word or another Office app, go to File > Account > Update Options > Update Now. If no Office app opens, skip this step.

Microsoft’s current update procedure is available in How to update Microsoft 365 or Office for Windows.

3. Uninstall and reinstall Microsoft 365 if the error persists

If Online Repair does not fix the problem, or the Click-to-Run folder is missing important files, reinstalling Microsoft 365 is safer than copying APIClient.dll from another PC.

1. Open Control Panel > Programs > Programs and Features.
2. Select Microsoft 365 or your Office suite and choose Uninstall. If the normal uninstall fails, use Microsoft’s official uninstall troubleshooter.
3. Restart the PC after removal.
4. Sign in at Microsoft’s Microsoft 365 Apps page with the Microsoft account, work account, or school account associated with your license.
5. Select Install apps and complete the installation.

Microsoft notes that uninstalling Microsoft 365 removes the applications but does not delete the documents, workbooks, or presentations you created. See the official uninstall instructions and Microsoft 365 installation instructions.

4. Run DISM and SFC if Windows shows other DLL errors too

If APIClient.dll is only one of several missing or corrupted-file errors, check Windows itself. Microsoft recommends running DISM before System File Checker.

1. Search for Command Prompt, right-click it, and choose Run as administrator.
2. Run DISM.exe /Online /Cleanup-image /Restorehealth and wait for it to finish.
3. Then run sfc /scannow and let the verification reach 100%.
4. Restart the PC and test Microsoft 365 again.

These commands and their order are documented by Microsoft in its System File Checker guidance.

What you should not do

• Do not download APIClient.dll from a DLL repository. You cannot verify that a third-party copy matches your Office build or is trustworthy.
• Do not copy the DLL from a random second PC. Microsoft 365 builds and architectures can differ.
• Do not use regsvr32 /u as the standard fix. The old advice to unregister this DLL is not part of Microsoft’s current Office repair procedure and can distract from the actual Click-to-Run problem.
• Do not rename backup DLL files unless Microsoft Support specifically directs you to. Old forum workarounds may not apply to your current Office installation.

If APIClient.dll exists but Office still says it is missing

The message does not always mean the file is literally absent. A damaged Click-to-Run installation, mismatched Office files, or broader Windows corruption can prevent the file from loading correctly. In that case, use Online Repair rather than deleting or replacing the DLL manually.

A Windows 10 support note

Windows 10 reached end of support on October 14, 2025. Microsoft says Microsoft 365 Apps can continue receiving security updates on Windows 10 through October 10, 2028, but it recommends moving to Windows 11 because running Microsoft 365 on an unsupported operating system can lead to reliability and support limitations. See Microsoft’s Windows 10 and Microsoft 365 Apps support policy.