Yeah, he's dead serious.
Pras is a huge New England Patriots fan -- and tells TMZ Sports he's used some of his hip hop fortune to fund some massive sports betting over the years ... including a $100k bet on Super Bowl LI.
"In the Super Bowl this year, at the end of the 3rd quarter, my friend called me and bet me $100,000 'cause he thought it was over ... and I took him on, and I won $100,000."
The only catch -- Pras says his boy still owes him $30K ... and he's callin' him out to pay up!
Meantime, how 'bout sending Brady an autographed album or something? Least you could do, right?