The 17 year-old will work across the Under-18 and Under-21 squads at Saints following his arrival from Greenhous Meadow on a three-year professional contract.
The goalkeeper has received international recognition at both under-16 and under-18 level for England, making his debut for the Under-18s in a game against Italy last year.
The youngster, who signed his first professional contract with his local club in early 2014, also attended a goalkeeper training camp with England earlier this month.
Academy Manager Matt Hale was pleased to welcome Lewis to the club. “He’s got a good background and with the staff and the environment we’ve got here, he’s got every opportunity to fulfil the potential we’ve seen in him,” he said.
“He’s still eligible to play for the Under-18s but he’s here on a longer-term contract where we want to see him grow. No doubt his aspiration, as well as ours, will be to push on and try and get into the Under-21s as soon as possible.
“The calibre which Harry comes with provides us with an exciting time ahead. He has played for England and that’s our benchmark of where we need our young players to be now.”
Lewis added: “I think just looking round the place you can see it is a fantastic football club with brilliant facilities and excellent coaching staff.
“I think it was a no-brainer really for me to make this move. I’ve been at Shrewsbury since I was eight and it is my hometown club. I’m always going to be close to them but when an opportunity like this comes around you don’t say no.
“I’m here to get better as a footballer and get as many chances as I can. That’s the most important thing for me.”