Fa'amatuainu will add greater depth to the Scarlets squad and is expected to challenge the likes of Welsh youngsters Rob McCusker and Josh Turnbull, as well as the returning Dafydd Jones, for a spot in the starting XV.
A fans favourite at the Recreation Ground, Fa'amatuainu currently has 11 caps for his country and can also play in the second row as well as any of the back-three positions.
The region have been quiet in the transfer market this summer but head coach Nigel Davies believes his new signing will bring an extra energy to the Parc-y-Scarlets for the 2010/11 campaign.
"Jonny will offer us some exciting options not only as a flair player but because of his flexibility to be able to play across the back and second row," said Davies.
"We've been talking to Bath for a while now - the move fits well with us as we've been looking to bring in new blood and certain types of player into our forwards.
"Jonny's obviously hungry for more game time and his move fits with Bath's squad make-up next season and the new signings they've made this summer.
"He's been down a few times to meet with us and see our set-up and was clearly impressed with the training facilities and the stadium at Parc y Scarlets and is looking forward to making an impact here amongst the team and with our supporters.
"We look forward to welcoming him into the squad and seeing what he's capable of doing for the Scarlets next season.
"This gives us more competition and energy in the group and that can only be a good thing."