The 22 year-old, 6ft and 18 stone loose head prop, played age group rugby for Wales at U16 and U18 level before joining Cardiff and making his full debut for the club aged just 18.
A strong scrummager, he went on to become part of the Wales U21 squad and made his first appearance for the Blues against Munster at Thomond Park in 2006.
Saracens Director of Rugby Brendan Venter said his countryman (Visagie) had been impressed with the young prop after seeing him in action.
Venter said: "Rhys is a strong scrummager, who has proved his worth with several outstanding performances recently. Cobus Visagie has identified his talent, and we are confident Rhys will play his part in a strong complement of front row forwards at Saracens next season."
Gill will join the likes of new signings Carlos Nieto, Richard Skuse and Frik Venter in the Saracens squad next season.