He'll join up from Edinburgh for the start of the 2015/2016 season.
The New Zealand-born scrum-half joined Edinburgh from Super Rugby side NSW Waratahs in 2013 and scored a try on his debut against Ulster, after coming off the bench at the Kingspan Stadium.
He played for the New Zealand U20s in the 2008 IRB Junior World Championship, where he kept current All Blacks scrum-half Aaron Smith on the bench throughout the tournament.
Grayson qualifies to play for Scotland through his Glasgow-born grandmother and was called up to the Scotland squad for last year's summer tour.
He made his debut for Vern Cotter's side as a substitute in the win over Canada in June.