Law enforcement sources tell TMZ Sports the incident happened at the Millennium Hotel in New York.

We're told police responded to a call of a dispute around 6:45 AM ... and at the scene, cops observed a laceration above the 26-year-old woman's right eye and noticed there was blood on her face.

The woman -- who also complained of neck pain -- claimed KPJ struck her multiple times throughout her body during the physical altercation. EMS responded to the scene ... and the girlfriend was taken to a nearby hospital for evaluation.

Porter is currently behind bars in New York on assault and strangulation charges.

Our sources claim Porter's girlfriend was upset over how long he stayed out earlier in the night ... so she locked the doors to keep him from entering their room when he got back.

We're told KPJ eventually got into the room with help from hotel security ... and the alleged altercation ensued minutes later. Witnesses say the girlfriend ran out to the hallway of the hotel and screamed for help.

KPJ -- the 30th overall pick out of 2019 NBA draft -- has played for the Rockets and Cleveland Cavaliers so far in his career ... averaging 19.2 points and 5.7 assists a game for Houston last season.