The 22-year-old center back has put pen to paper on a four year deal after an impressive loan spell.
Harwood-Bellis spent last term at the club on loan from Premier League champions Manchester City.
“It’s no secret how much I enjoyed being here last season, so I’m delighted to officially become a Southampton player ahead of the new season,” said Harwood-Bellis to club media.
“The feeling of bringing this club back to the Premier League and playing in front of these fans week-in and week-out has been amazing.
“I’ve learnt a great deal over the past two years and developed so much as a player and as a person. To have the opportunity now to represent this club in the Premier League is really special and I can’t wait to get going and to see all the lads again.”