Harris, 27, who was dressed in a gray business suit, told the judge he understands the terms of the agreement.
He pleaded guilty to possession of an unregistered firearm, possession of machine guns and possession of a firearm by a convicted felon.
Harris is one of pop music's most successful artists. His sixth album, "T.I. vs. T.I.P.," was released July 3, debuting at No. 1. He appeared in the 2007 film "American Gangster," which starred Denzel Washington and Russell Crowe.
He was arrested Oct. 13, just blocks away and hours before he was to headline the BET Hip-Hop Awards in Atlanta.
Harris was charged with possession of unregistered machine guns and silencers, as well as possession of firearms by a convicted felon. He faced a maximum of 10 years in prison and a $250,000 fine for each count.
He was allegedly trying to buy unregistered machine guns and silencers. He initially pleaded not guilty, and has been under house arrest since he was released on $3 million bond on Oct. 26.
U.S. District Judge Charles Pannell Jr. must approve the deal.
Harris, who is co-CEO of Grand Hustle Records, grew up in Atlanta. His first taste of success came with his 2003 album, "Trap Muzik." In 2004, warrants were issued for his arrest on probation violations for a drug conviction, and he was sentenced to three years behind bars.