Sarries coach Buck Shelford confirmed the news to Planet Rugby, although an official announcement has not yet been made by the London club, who last weekend qualified for the semi-finals of the Parker Pen Challenge Cup.
The 25-year-old had been linked with a code switch for some time after contract talks with Bradford Bulls broke down, the Yorkshire club being the current world club champions, and the former home of another England Sevens player, Gloucester's Henry Paul.
McAvoy is a big strong runner, who captained the Great Britain Academy squad back in 1996.
Making his full Test debut later that year, his subsequent trophies have included the Super League title and Challenge Cup, as well as that World Club Championship.
He was quoted earlier in the season as saying: "I have never played Union at any decent level but I have played Sevens, and I think I could make the switch."
His Sevens experience so far included a blistering performance for Bradford Bulls in the 2002 Middlesex Sevens, when the league side swept aside all the Zurich Premiership competition in spectacular style on their way to winning the trophy.
By Mark Smith