The club is investigating the incident which happened in a local pub on Wednesday night, according to the Bath Chronicle.
Eye-witnesses told the paper of "disgusting" scenes and said that Henson was punched after being told to "calm down" by players.
The player who punched him was unnamed.
The police were called after a complaint about "aggressive behaviour" in the pub but did not go to the premises as the Bath group left the pub while the call was being taken.
A witness told the Chronicle that Henson "went out like a light for about five minutes."
"He just lay there like a lump, right on the floor."
Henson only joined Bath last month following London Welsh's relegation from the Aviva Premiership.
The 31-year-old has been blighted by back and calf injuries over the last season, which have hindered his hopes of re-establishing himself among the league's best players after a turbulent few years.
Bath are Henson's fifth club in the space of two years as he seeks to revive a career that promised much in his youth. The fly-half or centre has won two Grand Slams but has not represented his country since winning the last of his 33 caps in 2011.
Bath chief executive Nick Blofeld said: "We are speaking to all those involved to clarify the situation. When we have all the facts, we will make any appropriate statement.”
- Sports & Recreation