Gronk never spent a dime of his football money til' NOW!

ROB GRONKOWSKI has been in the NFL since 2010.  And in all that time, he never spent a DIME of the money he's earned playing football.  He stashes that away and lives modestly off his endorsement dough.

But that just changed. 

On the web show "Kneading Dough", Gronk said he recently bought himself something expensive with his earnings.

Gronk was never a jewelry guy, but his friend had a chain that he thought was really cool.  The guy let him wear it to a party, and it, quote, "made me feel good."

So he decided that if he, quote, "hit all my incentives, if I do everything I need to do, get all my bonuses, put all the work in," then he'd treat himself to the chain.  And he did.

He even pulled it out on the show and said, quote, "I love this puppy . . . Now I know why people got jewelry.  Now I understand why."

Photo: Getty Images