Check your tax return or tax documents
Look for your Social Security Number if you are an individual taxpayer in the U.S.
Look for your Employer Identification Number if you are a business
Review official letters from the tax authority
Check prior tax filings, W-2s, 1099s, or business registration documents
Log in to your tax authority online account
Contact the tax authority directly
Ask your employer, accountant, or payroll provider
Review your business license or incorporation records
Use the TIN assigned by your country’s tax agency