Lewis has signed a new three-year-deal at St Mary's, with the young goalkeeper also joining Dundee United on loan for the coming season.
Saints keeper coach Dave Watson said: “It was fantastic for Harry last season to get three games in the FA Cup and that experience has put him in the shop window for us to get the opportunity to get him out on loan
“At his age, he needs to go and get game exposure. It's a tough challenge to go to Dundee United in the Scottish Championship.
“He'll be playing men's football week in, week out and he's got to earn the right to be in the team. They're going to be chasing promotion back to the Scottish Premier League.
“It'll be a challenge but if he can overcome that then it's a whole different set-up when he comes back."