The 19-year-old signed a three-year-contract with Southampton prior to his switch to the Tangerines where he will challenge for the number one spot.
United have already brought in one goalkeeper, Turkish stopper Deniz Mehmet, with current number one Cammy Bell expected to make the move to Kilmarnock if reports are to be believed.
Lewis, who stands at 6ft3in, made three appearances for the Premier League side in last season’s FA Cup campaign, keeping a clean sheet against Norwich City. Although he did let in five against Arsenal.
He’s played a number of games for the clubs youth sides following a six-figure move from Shrewsbury Town for a six-figure fee in 2015, and has represented England at under-18 level.
Lewis told the club’s official website: “I am keen to get started here at United. I am excited at the prospect of being involved in first team football for this Club and hope to show my ability at this level.
“I believe I am joining a winning team that will be challenging at the top of the league and want to play my part in helping the Club secure the title.”
Lewis is manager Ray McKinnon’s eighth summer signing, joining the aforementioned Mehmet, as well as James Keatings, Billy King, Thomas Scobbie, Paul McMullan, Patrick N’Koyi and Jordie Briels.
McKinnon said: “Harry comes here as one of the top young goalkeeping prospects in English football. I am delighted to bring him to United and I expect him to challenge for the starting position.
“It says so much about his ambition and desire that he has chosen to come up to Scotland to get first team involvement. It is a coup for us to bring him to Tannadice and I am positive we will hear much more about him during his career.”
Meanwhile, United have fans have showed their excitement for the season with more than 4,000 season tickets sold three weeks ahead of the start of the league campaign.