YK Osiris allegedly beat the crap out of his girlfriend at his own birthday party ... and now he's being held on a felony charge.
Law enforcement sources tell TMZ ... the singer was arrested Monday in Atlanta on a felony charge of aggravated assault by strangulation.
According to the police report ... YK Osiris' girlfriend told cops back in September she got into a physical altercation with the singer at his bday bash, which ended with him choking her and biting her face.
The woman says she confronted Osiris after seeing an image on his phone of another woman wearing nothing but a towel. The girlfriend claims they started yelling and Osiris said, "I am going to slap the s**t out of you."
Osiris' GF told cops he chased her up a set of stairs into a bathroom and that's when he choked her and bit her face. The woman claims she bit his lip to get him off her, and the fight was eventually broken up.
The police report says the woman had a noticeable mark under her left eye, where she claims she was bitten.
Osiris was denied bond after his arrest, and he's got a preliminary hearing later this month at the Fulton County Courthouse.
We reached out to the singer's team ... no word back, so far.