Check your tax return documents, such as your filed returns, notices, or correspondence from the tax authority
Look at your tax registration certificate or taxpayer identification card, if you have one
Review payroll records, pay stubs, or employer tax forms that may show your taxpayer ID
Log in to your online tax account with the relevant tax authority
Contact the tax authority directly and verify your identity to request your taxpayer ID
Ask your employer, accountant, or tax preparer if they have your taxpayer ID on file
Check official letters, invoices, or business registration documents that may include the number