Henson, 30, has been without a club since April after being sacked by Cardiff Blues over his drunken behaviour on a flight returning from a game in Glasgow. But Henson worked with Exiles coach Lyn Jones during his time at the Ospreys and has been given the latest in a series of chances to get his career back on track.
Jones said: "Gavin has a great talent but over the past three or four seasons he has not realised his full potential for a number of reasons that are well documented."
He added: "I sincerely hope and believe he can now draw a line under that period and move forward again with London Welsh."
Given the player's history the move will be considered by many to be a major gamble for London Welsh as they prepare to embark on their first top-flight campaign.
Despite playing an integral part in Wales' 2005 and 2008 Grand Slam campaigns, and touring New Zealand with the Lions in 2005, Henson has yet to deliver consistently on the glimpses of class he has shown on occasions.
But Jones, with whom Henson won two Celtic League titles and an Anglo-Welsh Cup between 2003 and 2008, believes the right environment can bring out the best in the wayward star.
He said: "What's important for Gavin is that he concentrates 100% on his game and learning how to become the best he can possibly be again. I think he can add a great deal to our game and we can also do a lot for him.
"We all appreciate the skills that he can bring to a game but what team-mates and coaches will be looking for is consistency of that ability. That's the challenge for Gavin, to hold his concentration fully on rugby as he did when he started his career, when rugby was first, second and third on his list of priorities."
The Blues were Henson's fourth club in little over a year after being released by the Ospreys in 2010 and having short stints at Saracens and Toulon. His spell in France had also been marred by an alcohol-related incident, with the club suspending him for a week after a nightclub altercation with two team-mates.