Robert Guerrero pleaded to the lesser charge in a Queens courthouse and was ordered to pay a $250 fine and complete 50 hours of community service.
Guerrero, a former featherweight champion who lost a welterweight title bout to Floyd Mayweather Jr. this month, lives in Gilroy, Calif., and will be able to perform the community service in his home state.
The 30-year-old fighter presented a locked gun box during check-in at John F. Kennedy International Airport on March 28 while on his way to a Las Vegas training camp for the Mayweather fight. The box contained an unloaded .40-caliber handgun and three unloaded magazines. The gun was registered in California but was illegal in New York.
Guerrero said in an emailed statement he was pleased prosecutors considered his case "carefully and resolved it fairly."
"I never intended to violate New York law, but I know that ignorance of the law is not an excuse," he said in the statement. "Lesson learned. I'm happy this incident is behind me and looking forward to returning to the ring as well as serving my community as ordered by the court."
Guerrero had called out Mayweather after winning a fight late last year, saying, "Pretty Boy, let's do it." But he lost to Mayweather on May 4 in a unanimous 12-round decision.
The fight was the first for Mayweather on Showtime under a six-fight deal that lured him from his former home at HBO. Mayweather landed thudding right hands and bloodied Guerrero's face.