Sydney Roosters second-rower Ryan Matterson on a three-year deal.
The 23-year-old has played 57 first-grade games since making his debut for the Sydney Roosters in 2016.
"Ryan is a talented young man and we're very pleased to have him join the club next season," Wests Tigers coach Ivan Cleary said.
"He has developed in to a quality forward since his debut and we're looking forward to seeing him grow further at Wests Tigers over the next three years."