Captain Rattan, in a turban and full beard, graduated on Monday, Mar 22 at Fort Sam Houston in San Antonio.
Speaking to reporters, Rattan said, "I'm feeling very humbled. I'm a soldier. This has been my dream."
Rattan had to get a waiver from the Army to be allowed to serve without sacrificing the unshorn hair mandated by his faith.
Rattan, an immigrant from India who arrived in New York as a teenager, said it was important for him to serve a country that has given him so many opportunities.
The Army made an exemption to a uniform policy that has effectively prevented Sikhs from enlisting since 1984.
Rattan and Kamaljeet Singh Kalsi, who will attend basic training this summer after completing an emergency medicine fellowship, are the first Sikhs to receive exemptions in more than 25 years.