Concussion forces Springbok to retire
Hargreaves made 79 appearances for the Men in Black since arriving at the club from Super Rugby franchise the Sharks in 2012.
He skippered Saracens to the 2014/15 Premiership title in his first season as captain.
"It's been a very tough decision to make," the 30-year old Hargreaves said, about the decision to hang up his boots.
"I've suffered a number of concussions in the last two seasons and after seeking medical advice I've come to the conclusion that the risk is outweighing the reward.
"I have a young family, I have to make decisions that are right by them and although this was a hard decision for me personally it's one that I feel is the right one to make.
"The club have been amazing ever since I walked through the doors, but especially over the last 12 months, when I've been suffering from injuries and concussion.
"They have been a constant source of support in a really emotional and difficult time.
"The first person I spoke to about this was Mark [McCall] and he was completely supportive of my decision.
"This club puts people first and their interests throughout have been: 'What's best for me and my family' and I'm extremely grateful for the way they have handled the situation."
Hargreaves added: "My period at Saracens has been the best four years of my life – It's been an absolutely phenomenal time.
"Everyone at Allianz Park; the players, the coaches, the backroom team and the fans, have all contributed to this incredible experience.
"It's an experience that I didn't expect when I arrived, but I leave as a Saracens man to the core.
"My time in South Africa was fantastic, it was great to play for the Sharks and be capped by the Springboks but this time at Saracens has been my highlight.
"I've grown as a player, but more importantly, I've grown as a human being."
Saracens Director of Rugby Mark McCall said the welfare of the players is of paramount importance.
"We are fully supportive of Alistair's decision," McCall said.
"Since arriving in 2012 Alistair has developed into one of the most influential players within the squad.
"He has played a key role in our recent success with his character, hard work and leadership coming to the fore over the last four seasons. We wish Alistair, his wife Carmen and their young son the very best for the future."
