Grace is the seventh Scarlets signing after prop Corey Domachowski, locks Harvey Cuckson and Alex Groves, hookers John McKee and George Roberts and back-three player Isaac Murray-Macgregor.
Scarlets’ departures include Marnus van der Merwe, Alec Hepburn, Tom Rogers, Jake Ball, Max Douglas and Jarrod Taylor.
Skipper Josh Macleod, Fletcher Anderson, Gareth Davies, Jac Davies, Dan Davis, Sam Lousi, Will Evans and Harri O’Connor have signed new deals.
“Scarlets is a proud rugby club with a great history and tradition and that was something that appealed to me,” said Cullen.
“I am looking forward to coming over, learning, growing my game and doing everything I can to contribute to the club, its rugby group, the supporters and its legacy.”
Scarlets open their URC campaign with a home Welsh derby against Cardiff on Saturday, 26 September.
