The Selective Service System keeps a list of men who could be called to the military if the US ever had a draft again. There is no draft now. Registering is just giving your name and address.
Almost all men who live in the US and are 18 through 25, including:
Green card holders (including conditional green cards).
Men without legal status.
Men in the US on a valid nonimmigrant visa (for example F-1, J-1, H-1B, H-4, L-1, L-2, B-2) usually do not register while they keep that status.
Register within 30 days of your 18th birthday, or within 30 days of arriving or becoming a permanent resident if you are 18 to 25. After your 26th birthday you can no longer register.
Women do not register.
Open sss.gov and choose Register.
Enter your name, date of birth, address and Social Security number. If you do not have an SSN yet, you can register and add it later, or wait a few weeks for it.
Save the confirmation number.
A law passed in late 2025 says registration will become automatic for most men, using government records, starting around December 2026. Until you receive a confirmation, register yourself. It takes five minutes and is free.
Not registering can block some federal jobs and some state benefits.
When you apply for citizenship, USCIS asks about it. Not registering on purpose can be a problem for the application.
Thinking it means joining the army. It does not.
Forgetting because you arrived at 22. The 18 to 25 rule applies to the age you are now.