til 6 AM!!
James and the rest of the world champion Miami Heat hit up the LIV Miami nightclub at the Fontainebleau Hotel ... where LeBron was drinking out of a 15-liter bottle of Ace of Spades champagne ... which usually run around $75k.
Sources at the club tell us ... LeBron grabbed the mic ... and rapped along with the music all night long ... while a bunch of his teammates puffed away on cigars.
At one point, 'Bron rapped along with the song "Riot" by 2 Chainz while LMFAO rocked out on a nearby stage.
After the Heat defeated the OKC Thunder in Game 5 last night ... a relieved LeBron exclaimed, "It's about damn time."
Thanks to our friends at WorldRedEye.com for the footage.