The 30-year-old South African lock made 79 appearances for Saracens after joining from Super Rugby franchise Sharks in 2012 and led the side to the 2014-15 Premiership title in his first season as captain.
"It's been a very tough decision to make. 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," Hargreaves said.
"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."
Hargreaves represented South Africa at under-19 and under-21 level and was capped four times by the senior team.
Earlier this year, Saracens appointed centre Brad Barritt as captain and the centre led the side to the Premiership and European Champions Cup titles while Hargreaves was sidelined through injury.
Together with former team mate Chris Wyles, Hargreaves has launched a brewery and told a British newspaper earlier this week that the pair were considering opening pubs and creating more beers.
(Editing by John O'Brien)
