Boxing star Devin Haney was arrested on Thursday morning following a night out in Hollywood ... after cops say they found a semi-automatic handgun in his car during a routine traffic stop.
The 30-0 fighter -- who's coming off a huge win over Vasiliy Lomachenko -- was out at Catch L.A. on Wednesday ... when just minutes after he left the hotspot around midnight, law enforcement tells TMZ Sports cops pulled over his ride due to an unsafe lane change and a failure to use turn signal.
According to authorities, Haney was not driving ... but the person who was told officers during the stop there was a firearm inside of the ride.
Cops say they then got Haney and his security out of the car ... and when they searched the vehicle, they say they found a semi-automatic handgun under the driver's seat.
Law enforcement tells us ... no one took ownership of the gun -- and since the car was registered to Haney, he was arrested and booked for felony possession of a concealed weapon.
Jail records show he was released at 8:12 AM Thursday on $35,000 bail. He's due in court for a hearing on the matter next month.