The 28-year-old 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.
"I am genuinely delighted to get this opportunity to play for Saracens," said Henson.
"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."
The London-based side struggled in the opening two rounds of the Heineken Cup, losing away to French Top 14 Champions Clermont Auvergne and then former European Champions Leinster at Wembley.
Saracens may be able to call upon Henson for the Pool stages after the injury to fly-half Derick Hougaard which ruled him out for the season.
Each side are able to nominate one additional player to their 38-man original squad to replace an injured player and Henson may be called upon when the Heineken Cup returns in December.