Brittney Griner -- Skateboard Clause In WNBA Contract ... 'I Know What's At Stake'

Before Brittney Griner signed her fat new contract as the #1 overall pick in the 2013 WNBA draft ... she had to make a promise to her team -- I'LL TONE DOWN MY SKATEBOARDING HABIT.

"I agreed with the Mercury that I would only ride in a controlled environment," Griner tells TMZ ... "No tricks or hill bombing."

Besides being the most dominant female basketball player to come out of college in years, Griner is an avid skater ... who's been riding since she was a little kid.

"I know what is at stake, so I'll just be cruising. I'll get my adrenaline rush on the court."

Griner -- who's 6'8" and DUNKS -- caused a major uproar during her college career at Baylor when she broke her wrist while riding in a parking structure.

"Before that, I had never been injured," Griner said ... explaining, "[I was] going too fast."

"Once my basketball career is over though, I will be building a skate ramp in my yard, that's for sure."