Foltynewicz, whose fastball has been clocked at over 100 mph, is the fourth-rated prospect in Houston's system according to Baseball Prospectus.
He has mostly worked as a starter in the minors, but will pitch out of the bullpen in Houston.
The 22-year-old went 7-7 with a 5.08 ERA in 21 appearances, including 18 starts, for Oklahoma City this season. Foltynewicz was the 19th overall selection in the 2010 draft.
He says he's excited about "getting my chance to prove I belong here" and says the key to success will be limiting his walks and attacking hitters.