The Cleveland Cavaliers Rickrolled 20k Fans!

LeBron James and company pulled an awesome gag on fans Friday night ... "Rickrolling" the sell out crowd before dominating the Miami Heat.

The Cavaliers held '80s Night at The Q in Cleveland ... and when it came time for starting lineups, the scoreboard switched from its standard pump-up vid to the prank.

LeBron with long hair looks ... good? And Khloe Kardashian might want Tristan Thompson to bring home that Jheri curl wig.

Just for giggles, check out LeBron Jr. after the game -- the 12-year-old sunk a half-court shot like it was nothing. Get that kid an awful 80s outfit.