Hours after officially signing his L.A. Lakers contract, LeBron James and his wife celebrated in Malibu ... with his OLD Cleveland Cavs teammate Tristan Thompson (and Khloe Kardashian!).

The foursome -- along with a ton of security and others -- hit up Nobu in Malibu ... one of the most expensive sushi joints in town.

Not like price matters (LeBron just signed a $154 million contract, Tristan is in an $82 mil contract and Khloe is rich as hell too).

So, why is Bron out with the old teammate instead of bonding with his new Lakers squad? Could be because the Lake Show is in Vegas at the moment competing in the NBA Summer League.

Then again ... it's not like LeBron couldn't have flown out (or flown them in) for a first official dinner together.

... Or maybe he just really likes Tristan and Khloe?!

On that note, TT and KK were ALL OVER each other in the car -- kissing and hugging up ... and posting it on social media for everyone to see.