Hargreaves has made 79 appearances in four seasons with Saracens, leading them to the 2014-15 Aviva Premiership title as club captain.
“It’s been a very tough decision to make,” Hargreaves said in a statement issued by the club. “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.
“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. I’ve grown as a rugby player but more importantly, I’ve grown as a human being.”