The 40-year-old had been in charge at Alloa Athletic since December, but was unable to prevent them from being relegated to Scottish League One.
Alloa won their opening 10 matches of this season in all competitions, though, resulting in St Mirren turning to Ross after failed bids to bring Billy Davies, Neil Warnock and John Hughes to the club.
"I'm delighted to be named the new manager of St Mirren Football Club. I have fond memories of my time in Paisley as a player and can't wait to get started," Ross told the club's official website.
Saints chairman Gordon Scott added: "I was very impressed with Jack when we got a chance to meet and am delighted to have him on-board as our new manager. I look forward to working alongside him as we look to bring success to the football club."
Ross, who spent two years with St Mirren during his playing career, replaces Alex Rae at the helm and takes over a club bottom of the Championship without a win from eight league games so far this term.body check tags ::