The 27-year-old, who can play wing and full-back, is currently in camp with Scotland as the prepare to fly out for a two-Test tour of Japan at the weekend and leaves London Irish, who were relegated from England’s top flight last season.
The former Glasgow player, who has 21 caps, said: “When I was approached about joining Saracens I had no hesitation in accepting the opportunity to join up with their squad.
“The results they have achieved this season speak for themselves but what really appealed was the burning desire from the coaches to continuously improve and evolve. I think the surface at Allianz Park will suit my game and I look forward to meeting up with my new team-mates in the summer.
“I’ve really enjoyed my time at London Irish this season despite the obvious disappointment in getting relegated. There are some great people and players there, and I am sure that the talent within the squad will see them back in the Premiership in 2017/18.”
Saracens also announced the signing of South Africa pro Vincent Koch from the Stormers yesterday and director of rugby Mark McCall: “Having already announced a number of signings earlier this year, we are pleased to have finally secured each of our targets.
“We identified Vincent as a player who could make a significant contribution to the squad at tighthead prop and at 26 he is a player with huge potential.
“Sean is a highly experienced and proven international three quarter, and we believe that he will flourish here.
“Both players are exactly the kind of characters we think will thrive in our environment, and we are confident they will become key players for Saracens.”