UFC president Dana White confirmed the matchup Tuesday in a text to The Canadian Press.
"Thanks for all your support!! I'm a happy man!" tweeted Grant, a 29-year-old from Cole Harbour, N.S.
"You earned it bro," White tweeted back.
Grant (21-5) had been slated to meet champion Benson (Smooth) Henderson last Saturday at UFC 164 in Milwaukee. He earned the title shot in May by knocking out Gray (The Bully) Maynard in the first round at UFC 160.
But a concussion forced him out of the Henderson fight, leaving Pettis (17-2) to make the most of the opportunity. The former WEC champion submitted Henderson in the first round after lashing his body with a series of leg kicks.
After beating Henderson, Pettis called out featherweight champion Jose Aldo, saying he would fight for either belt.
Grant is 5-0 since dropping down to 155 pounds. He went 3-3 as a welterweight.
There is no word yet on a date for the Grant-Pettis title fight.