Randell, who won 51 Test caps in the back row, is just one of an experienced crop of internationals to have fallen out of favour with coach John Mitchell.
The 28-year-old had been in negotiations with Saracens since the end of the last English season, but wanted to delay any firm decision on his future until he was certain not to be included in New Zealand's World Cup plans.
Although the 30-man squad is not officially announced until next month, Randell decided to bring his Super 12 career with the Otago Highlanders to a close and plough a new furrow in the Zurich Premiership.
"I'm very excited about the next phase of my career and can't wait to get settled in at the club," said Randell, who will link up with his new team-mates in August for the start of pre-season training.
"I know how ambitious the club is and I am looking forward to playing my part in getting the club back to where they belong.
"The Zurich Premiership has become one of the toughest leagues in the world and I'm looking forward to this new challenge."
Saracens have undergone something of an off-season overhaul with coach Wayne Shelford shifted out after just one season in charge.
Rod Kafer signed up from Leicester as player and backs coach, but chief executive Mark Sinderberry has been searching the globe for a new head coach.
ACT Brumbies forwards coach Laurie Fisher spurned the chance of a move to London, preferring instead to work towards one day taking over the reins at the Brumbies.
Former Australia coach Rod MacQueen was also linked with the post, but poured cold water on speculation both in England and Down Under.
Randell is the seventh player brought in by Sinderberry, after Kafer, Argentine international Emiliano Bergamaschi, Frenchman Raphael Ibanez, Alex Codling, Simon Raiwalui and Andy Kershaw.
"We're naturally very pleased to confirm Taine's signature," he said.
"And we're delighted that he has decided to begin the next phase of his career at the club.
"Not only does he bring a wealth of experience both on and off the field, he also has great leadership skills after leading both his province and country.
"Taine's signature brings an end to activity in the transfer market and we're now confident that we have a squad that can take the club forwards over the next few seasons."