NBA Star Joe Johnson -- Drops $10K in London Nightclub ... BRING THE CHAMPAGNE!

Another NBA star is doing his part for American diplomacy ... only this time, there's no homicidal dictator involved.

TMZ Sports has learned ... Brooklyn Nets star Joe Johnson hit up the China White nightclub in London Monday night and went on a champagne bonanza ... dropping close to $10k on bubbly.

Johnson and the Nets are in the U.K. to take on the Atlanta Hawks on Thursday as part of the NBA's plan to globalize the sport.

But 3 days before the game, Joe chalked up some big numbers at the nightclub ... buying 7 bottles of Dom Perignon Rose Luminous (at $1,200-a-pop), 2 shots of Sex on the Beach and some water.

Plus, Joe left a $960 tip on the bill ... making his bill for the night just shy of $10,000.