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Henson, who has won 33 caps, moves from London Welsh who were relegated from the top flight last season.
"I am thrilled to be joining Bath this summer, and I look forward to becoming part of what looks a very exciting squad," said 31-year-old Henson.
"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."
Henson had been linked with a move back to Wales this week with Newport-Gwent Dragons understood to be interested suitors.
Henson played just 11 times for London Welsh last season.
After leaving Ospreys in 2009, Henson has had short-lived spells with Toulon, Cardiff Blues before his one season with London Welsh.
But his recent career has been plagued by injury and discipline problems.
"We know Gavin's qualities and have seen him work extremely hard for his team and perform at a consistently high level last season," said Bath coach Gary Gold.
"He is still a very ambitious man and the prospect of him playing at the Rec is an exciting one."
