If you've already booked your trip and you're wondering how to add your SkyMiles number to an existing reservation, note that you can do this after the fact by accessing the “My Trips” tab on Delta's website.

Yes, you can add your Delta SkyMiles number to a reservation at any time before you fly, and even retroactively after the flight has landed. To add it before your trip, simply log in to the Delta website or "Fly Delta" app, navigate to "My Trips," enter your confirmation number, and look for the "Add Loyalty Program" or "Passenger Information" section. If you have already completed your travel, Delta allows you to request mileage credit for up to nine months after the flight date. You can do this through the "Request Mileage Credit" form on Delta's website. In 2026, it is highly recommended to add your number before check-in to ensure you receive your Medallion benefits, such as priority boarding or complimentary upgrades, which are processed based on the status attached to the ticket at the time of the upgrade window.