NFL's Pierre Garcon -- Legal Weed Could Be Great ... For My Pizza Biz!

Legal weed in Washington D.C. means legal munchies in Washington D.C. ... which could be HUGE for the pizza business ... and NFL star Pierre Garcon says he's ready for the boom!!!

The Washington Redskins wide receiver -- one of the founders of SpinFire Pizza -- just opened his 2nd location ... this one just miles from the nation's capital, where weed became legal back in March.

"Hopefully, it helps my business," Garcon joked with TMZ Sports ... "but hopefully, not too many people get in trouble with the marijuana in the league."

As far as the pizza business goes, Garcon says he's trying to catch up to Peyton Manning ... who owns 21 Papa John's locations in the Denver area -- and is CRUSHING it, btw.

"I'm trying to grow like he is!"