A routing transit number (RTN) or ABA number is a nine digit bank code, used in the United States, which appears on items such as checks that identifies which financial institution it is drawn upon. This code is also used by the Automated Clearing House to process direct deposits and other automated transfers. This system is named after the American Bankers Association, which designed it in 1910.

There are approximately 24,000 active routing and transit numbers currently in use. Every financial institution has one of these; it is a 9-digit number printed in MICR font at the bottom of checks that specifically identifies which financial institution it is associated with, and it is governed by the Routing Number Administrative Board which is sponsored by the ABA.

The primary purposes of the routing number are:

To identify the bank which is responsible to either pay or give credit or is entitled to receive payment or credit for a financial transaction.

To provide a reference to a designated presentment point of the bank at which the transaction can be delivered or presented.

For more information, see ABA Routing Number Content Blade.