Giancarlo Stanton -- Parties with $20k Champagne ... After Signing $325 Mil Contract!

Hours after Giancarlo Stanton signed his brand new $325 MILLION contract, the MLB superstar HIT THE CLUB and partied with a $20,000 bottle of champagne ... and TMZ Sports has the pics.

The Miami Marlins outfielder hit up FDR at the Delano Hotel in Miami .. where he was seen raging around 3 AM Tuesday morning with his boys ... and some very attractive ladies.

But the best part of the night ... when the guy who just signed the LARGEST sports contract in American history was GIFTED a $20k bottle of rare champagne.

It's called Moët Nectar Impérial Rosé Leopard Luxury Edition Methuselah -- and the 6 liter bottle is coated in a 22-carat gold leaf. It's one of only 60 bottles ever created.

So, who paid for the bubbly? We're told it was a gift from a well-known Miami party queen named "Julz" ... who just wanted to help Giancarlo celebrate.

So, the guy with the $325 million check gets free drinks. #RichGetRicher