EXCLUSIVE

Here's Giannis Antetokounmpo buying a MASSIVE teddy bear at Costco ... just two days before he signed his big ol' $100 MILLION contract with the Milwaukee Bucks.

Don't worry ... it only runs about $30 ... so, he's gonna be fine.

We're guessing the Greek Freak was getting some birthday shopping done at the Milwaukee-area super store for his girlfriend ... whose birthday was also Saturday.