The 29-year-old, who has won 23 caps for his country, spent seven years with the French side before arriving at Adams Park.
"I'm very happy to be here and I'm very excited," Bosch told Sarries' official website. "My first impression of Saracens is that I'm really impressed with the club.
"It's a good challenge for me. After seven years of living in France and playing in the Top 14 I've made the change to the Premiership and it's great to playing in the Heineken Cup – it's very exciting for me and I'm delighted to be here."
Saracens return to European action on Friday when they take on Toulouse at Wembley Stadium.