Petrino was to be introduced at a Monday afternoon news conference.
The 51-year-old Petrino replaces Willie Taggart, who left to become South Florida's coach on Saturday. Petrino had a 34-17 record at Arkansas and was 75-26 overall in eight seasons as a college head coach.
He was fired by Arkansas in April for a "pattern of misleading" behavior following a motorcycle accident. Petrino had an affair with former Razorbacks volleyball player Jessica Dorrell, whom he later hired as a football assistant, and gave her $20,000 in gifts. Petrino initially said that he was the only person on the motorcycle but later admitted to Dorrell's presence.
Petrino has been looking to get back into coaching since his dismissal. His name was mentioned recently in connection with several openings, including Kentucky and Auburn.
Petrino returns to the state where he began his head coaching career. He coached at Louisville from 2003-06, going 41-9 and leading the Cardinals to a 12-1 mark and their first-ever BCS berth in the Orange Bowl in 2006.
From there Petrino went to the NFL, where he had a brief 13-game stint in 2007 with the Atlanta Falcons. The Falcons stumbled to a 3-10 start before Petrino left for Arkansas, announcing his departure to players in a four-sentence laminated letter attached to their lockers.
Arkansas went 5-7 in his first season, but Petrino and the Razorbacks improved each year after that. They were to 8-5 in 2009, 10-3 with a Sugar Bowl appearance in 2010 and 11-2 with a Cotton Bowl bid in 2011.
He takes over a 7-5 Western Kentucky team that's headed to its first bowl appearance as an FBS school. The Hilltoppers will play in the Little Caesar's Pizza Bowl on Dec. 26 against Central Michigan. WKU defensive coordinator Lance Guidry was named interim coach on Saturday and will coach the team in the bowl game.