Telfair, a 21-year-old Brooklyn native, was charged with second-degree possession of a handgun and driving with a suspended license following the 3:53 a.m. stop on the Yonkers highway, Westchester County police said.
Telfair pleaded not guilty Friday at Yonkers City Court to the charges and bail was set at $7,500. The pair was due back in court May 17.
"The players on the Celtics have been explicitly warned, this year, to obey the team and league gun prohibition as well as state laws. We take this very seriously, and will act accordingly once the facts have been determined,'' Celtics owner Wyc Grousbeck said Friday night.
Celtics director of basketball operations Danny Ainge declined
Westchester County police spokesman Kieran O'Leary said Telfair was polite and cooperative.
"Our officers said he behaved like a gentlemen," O'Leary said.
According to Westchester police, Telfair was driving his 2006 Range Rover on northbound Bronx River Parkway when it was clocked speeding in a 45-mph zone.
A passenger, Al Eden Fuentes of Queens, was in the car with Telfair when it was stopped on the parkway, about five miles north of Manhattan. Telfair produced a Florida driver's license that had been suspended.
After stopping the Range Rover, the officer found a loaded .45-caliber handgun sticking out from under the front passenger seat, according to the Westchester County spokesman.
Telfair and Fuentes, 28, said they knew nothing about the gun, O'Leary said. Police still were investigating whether the gun was registered.
Police charged Telfair with a misdemeanor count of second-degree aggravated unlicensed operation of a motor vehicle and issued summonses charging him with speeding and driving without a license, both violations.
Telfair's brief NBA career has been marred by controversy, starting with his 2005 fine by the Portland Trail Blazers after a loaded gun was found on the team's private jet at Boston's Logan Airport.
The handgun was found in a pillowcase belonging to Telfair as the team plane was being prepared for a flight from Boston to Toronto. Telfair explained to local authorities that the gun belonged to his girlfriend and that he had inadvertently grabbed the wrong bag when leaving for the team's road trip.
In October 2006, Telfair reported a $50,000 chain was snatched off his neck at a Manhattan night club. He was unable to identify the men who stole the chain.
The 6-foot Telfair, a former New York schoolboy hoops legend, averaged 6.1 points and 2.8 assists a game this season for the Celtics.
Information from The Associated Press was used in this report.