Hargreaves, 28, succeeds Steve Borthwick, who retired at the end of the previous European season.
He told Saracens' official website it is a challenge he is looking forward to.
“The captaincy is a fantastic honour and it is a huge responsibility. I’m really looking forward to the challenge - we have a great bunch of players here, with lots of leaders.
“It’s a job I’m thoroughly looking forward to. We’ve got some great guys in the squad, some great characters, and it’s a lot of fun here - I can wait to crack on”.
Hargreaves joined Saracens from the Sharks in 2012. He played four Tests for the Boks in 2010 and 2011.