What is the USPS tracking number for a passport?

The USPS tracking number for a passport is a unique identifier assigned to the package when it is mailed, typically beginning with the letters "9" followed by a series of digits. You can find this number on the receipt provided by USPS when you send your passport application or when you receive your passport back. If you are expecting a passport, you can track its status online using this number on the USPS website.