You know you're at Leo DiCaprio's birthday when instead of a lame "Happy Birthday" sing-a-long ... you have Kanye West and 2 Chainz ripping the mic.

Leo celebrated his 39th last night at TAO Downtown in NYC with the rappers and a room filled with 100 of his closest friends -- mostly hot chicks. It's Leo, after all.

2 Chainz did lay down his version of "Happy Birthday" -- while Yeezus played "Good Life" ... "Gold Digger" ... and a bunch of other hits.

For his 40th ... Leo will probably bring 'Pac and Biggie back from the dead.