Fa'amatuainu, who can play anywhere in the back row or second row has signed a year-long deal at Parc y Scarlets to bolster Nigel Davies' Scarlets squad.
Fa'amatuainu has made 98 appearances for Bath since joining the club in 2006, scoring 15 tries.
Affectionately known as Jonny "F", he's been a Bath Rugby favourite earning himself a reputation for his creative play, hard running and tough tackling.
Born in New Zealand, Jonny has played for his native Manu Samoa seven times and made his international debut aged 22. He has played in all back five positions in the scrum.
His contract with Bath Rugby concludes at the end of the 2010-11 season and he remains under employment contract with Bath whilst on loan with the Scarlets next season.
His move to the Scarlets for the 2010/11 season's Magners League, Heineken Cup and LV=Cup action is subject to the transfer of his current work Visa with Bath to the Scarlets, which is expected to be rubber-stamped by authorities in the next fortnight.
Head Coach Nigel Davies said: "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.
"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."
