Agger spent eight years at Liverpool, making 232 appearances and winning the League Cup in 2012, but suffered with frequent injuries during his spell at the club.
He began and ended his career with Danish club Brondby, re-joining them in 2014.
"Thank you for your support. A great experience. It's sad, but it is the right decision to stop. I'm proud of my career," Agger said on his Twitter account.
Agger played 75 times for Denmark.
(Reporting by Martyn Herman; Editing by Toby Davis)
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