The 23-year-old Wales centre - whose outstanding performances in this year's Six Nations have put him in line for a British & Irish Lions tour spot - will stay with the Welsh region until the end of the 2008/09 season.
Henson said: "It is fantastic to be re-signing with the Ospreys. I never really wanted to consider any other Welsh side, let alone look outside of Wales, and its great to know I'll be staying for the next four years of my career.
"I think the side has grown immeasurably over the past year and that's been reflected in our performance.
"The Ospreys are now setting the pace in Welsh rugby, on and off the pitch, and becoming the model for others to follow - I'm just happy to continue playing my part in such a successful side."
Henson had been the subject of speculation about his future, with major English and French clubs reported to be interested in his services.
Ospreys rugby manager Derwyn Jones said: "We've worked hard to create an environment which has helped Gavin to thrive as a player and his recent performances are proof of that.
"He's an integral part of an extremely talented side that will be challenging for Europe's top honours next season."