Fa'osiliva, 27, turned out for Bristol last season and has represented Samoa at Sevens and full Test level.
The former Toulon man was part of the Samoa side which won the IRB World Sevens Series title in 2009-10 and he has gone straight into the Bath squad for the JP Morgan Asset Management Sevens Series.
He said: "I'm really excited to be coming to Bath and can't wait to get started with the Sevens on Thursday.
"It's a great form of the game, and one I'm obviously quite familiar with, so it's a perfect way for me to get my Bath career under way."
- Sports & Recreation
- IRB World Sevens Series