He previously had pleaded not guilty to illegal gun possession charges that carried at least 3½ years in prison upon conviction.
Lil Wayne remains free on bail while awaiting his February sentencing. The somber-looking rapper did not speak as he left court.
Police said a gun was found on his tour bus in Manhattan in 2007.
The rapper, born Dwayne Carter, won last year's best rap solo performance Grammy for "A Milli." His albums include "Tha Carter," "Tha Carter II" and "Tha Carter III."
His trial had been due to start Jan. 20. The judge had been holding a hearing on a debated DNA profiling technique used to tie the rapper to the gun.
Lil Wayne, 27, also is scheduled for trial in Arizona on felony drug possession and weapons charges. He has pleaded not guilty in that case, which arose from a January 2008 arrest at a U.S. Border Patrol checkpoint.
In New York, he politely answered the judge's standard questions with "yes, Sir" and "no, Sir" as he entered his plea to second-degree attempted weapon possession, a felony.
He acknowledged he had a .40 caliber semi-automatic loaded gun on bus.
When the judge warned that he would not be able to withdraw the plea as some people try to do, the rapper said, "I'm not one of those people."
Prosecutors said small amounts of DNA found on the loaded weapon connected it to the platinum-selling artist. Defense lawyer Stacey Richman said the gun wasn't Lil Wayne's, and the testing technique was too problematic to prove otherwise.
Police pulled over Lil Wayne's tour bus in Columbus Circle on July 22, 2007. They said they had seen and smelled marijuana smoke wafting out the door before the bus left a concert venue minutes earlier.
Police said that as an officer approached, the rapper tossed away a Louis Vuitton bag containing a gun.
The defense disputed officers' basis for searching the bus and noted that more than a dozen other people were aboard.