The 21-year-old spent last season with Burnley and played a crucial part in their return to the Premier League.
Harwood-Bellis made 32 appearances in the Championship as Vincent Kompany's side were promoted as champions with 101 points.
He also worked under the former City captain on loan at Belgian club Anderlecht and he has also had loan spells at Blackburn Rovers and Stoke City.
Harwood-Bellis has made eight senior appearances for Pep Guardiola’s side with his first-team debut in September 2019.
The highly-rated defender has also won caps for England’s Under-21s and captained them to their European Championship triumph in August 2023 along with City team-mates Tommy Doyle, Cole Palmer and James Trafford, who has since joined Burnley.
Everyone at Manchester City wishes Taylor the best of luck during his spell at Southampton.