The former Ospreys star was linked with a move to Newport Gwent Dragons, but has instead opted against a move back to Wales.
The 31-year-old has been blighted by back and calf injuries over the last season, which has hindered his hopes of re-establishing himself among the league's best players after a turbulent few years.
The 33-times-capped Henson said: "I am thrilled to be joining Bath this summer, and I look forward to becoming part of what looks a very exciting squad.
"I still very much want to be playing at the highest level, and Bath is a big club with big plans. I know what is expected of me and I can't wait to get started."