The 38-year-old, who retired from international cricket at the end of the most recent Ashes series, has signed a one-year contract at the County Ground in Taunton.
This will be the left-hander's fifth spell with a County Championship side, having previously played for Derbyshire, Leicestershire, Northamptonshire and Middlesex.
"I'm really pleased to be joining a club of the stature of Somerset. I have always enjoyed playing cricket in England and as soon as I heard that Somerset were interested I wanted to make it happen," he told the club's official website.
"There have been quite a few Australian players who have done really well down at Taunton and hopefully I can add my name to that list. I'm looking forward to the challenge and can't wait to get started."
In his 297 first-class matches to date, Rogers has scored 24,460 runs at an average of 49.81.