Wearing dark-blue jeans and a black sweater, the rapper said "guilty" and answered "yes" in a low voice to a judge's questions.
"Because of your guilty plea here today, you'll have a record of having committed a violent felony. This is a very serious matter," Manhattan state Supreme Court Justice Richard Carruthers told the rapper.
"This isn't a good day," Ja Rule said as he left the court. He declined to discuss the case.
Ja Rule's lawyer, Stacey Richman, had no immediate comment.
Specifically, the rapper admitted to a charge that involves attempting to have a loaded gun outside one's home or workplace.
Police said they found a loaded semiautomatic gun in a rear door of the $250,000-plus car after it was stopped for speeding.
In court Monday, Ja Rule was promised a two-year prison sentence. He's free until sentencing, on a date yet to be set. He is due in court Feb. 9 for an update.
The 34-year-old could have faced up to 15 years in prison, if convicted on the gun charge.
Known for his gravelly voice and collaborations with artists including Ashanti and Jennifer Lopez, Ja Rule scored a 2002 nomination for the best rap album Grammy Award with "Pain is Love." Born Jeffrey Atkins, he also has appeared in movies, including the 2001 film "The Fast and the Furious" and 2003's "Scary Movie 3."
Ja Rule, 34, was leaving the Beacon Theater when his car was stopped. Police said they smelled marijuana and found a .40-caliber semiautomatic gun in the rear driver's side door. He wasn't charged with drug possession, and a judge threw out the rapper's statement to police that he had "a little bit of weed" in his pocket.
Separately, rap big Lil Wayne also was arrested on a gun-possession charge after the same concert, which he headlined. At the time, the two had recently joined forces on a single called "Uh Oh."
Lil Wayne, born Dwayne Carter Jr., pleaded guilty last year to attempted criminal possession of a weapon. He admitted he had a loaded, semiautomatic .40-caliber gun on his tour bus, which was also stopped after the show.
The 28-year-old spent eight months in the city's Rikers Island jail complex, releasing an album, "I Am Not a Human Being," while behind bars. He was freed last month.