Ne-Yo brought all the ladies in his life on a special date night ... bringing out all four of his girlfriends to a club where he performed -- and, engaging in some PDA on the dance floor.

The rapper surprised the crowd at Slate NY, a popular nightclub in New York City's Flatiron District ... walking through the front door with all four of the women he's in a relationship with.

After the whole crew pulled up looking ready to party, Ne-Yo hit the stage ... playing some of his most well-known songs for the packed crowd.

The star belted out "Sexy Love," "Miss Independent," and "Give Me Everything" ... before giving the crowd his best a capella "Happy Birthday."

Later on, Ne-Yo was spotted on the floor ... locking lips with one of his GFs -- and grabbing a handful of her behind, too.

We've talked with Ne-Yo about his polyamorous relationship before ... and, he advocated to give polyamorous couples the right to marry -- though he admitted matrimony wasn't for him.

He was walking with two of his GFs at the time ... and, while he currently has four, he also told us there's no cap to the number of partners he'd have at any given time.