The 31-year old will join the Tigers from next season and is tied to the club until the end of 2021.
Tomau has played 263 NRL games, having begun his career at The North Queensland Cowboys where he made 170 appearances from 2009 to 2016.
Wests Tigers Coach Michael Maguire was thrilled to see Tamou come on board.
“James brings a wealth of experience to our forward pack and is a player who has experienced success at every level of the game,” Maguire told the club website.
“He has shown throughout his career to be a player who constantly improves not only himself but those around him, and I have no doubt he will play a key role in the development of our forward pack moving forward.”
Wests Tigers CEO Justin Pascoe added: “On behalf of all at Wests Tigers, I’d like to welcome James to the club and say how pleased we are to secure a player with his experience and talent for the coming two seasons.
“James has shown to be one of the leading forwards in our game and has generated extremely strong metres-per-minute this year, which will greatly complement our current squad.
“He is a highly-commendable man both on and off the field and will certainly have a strong impact at Wests Tigers in driving this club forward to where we all want it to be.”