The 29 year-old spent three years at United, having joined from bitter rivals Leeds United.
"Going back to any of your previous clubs is always a special moment," Smith told the club's official website. "And having got back into the Premier League, Manchester United away is a nice one to start with.
"We're looking forward to the Premier League knowing that we need to be fitter and stronger, and the lads have come back focused and in good shape ready for the new season.
"We're quietly confident inside the camp about what we can achieve and we've got to do ourselves proud and carry the momentum with us from last season."