Henson hasn't played since March 2009 after taking unpaid leave following a spate of injuries but will make his come-back with the side that finished second in the English Premiership last season.
He is currently appearing on BBC reality TV show Strictly Come Dancing and will combine his commitments until the show comes to end later this year but Henson has described his excitement at returning to the game.
"I am genuinely delighted to get this opportunity to play for Saracens," he said.
"They have created a special vibe at the club and, to be honest, just being around them has rekindled my enthusiasm for the sport. They're bold and innovative, and they're positive and ambitious.
"I know I still have so much to give to rugby, and I can't wait to get back on the field, playing for Saracens and hopefully for Wales.
"Now it's time to start working hard, getting back to match fitness as soon as possible and then to let my rugby do all the talking."
Saracens director of rugby Brendan Venter is delighted that Henson has signed for the London-based club.
"Gavin is a special talent and we are delighted he has chosen to launch the next phase of his career with us," said the South African.
"We believe he will enjoy our environment, on and off the field - that will be great news for our supporters and for Welsh rugby."