In a statement he said his body could not "put up with the rigors" of boxing.
"If I cannot give my family, my team, and the fans everything I have, then I should no longer be fighting," he said.
Earlier in the week Ward posted an image to social media showcasing titles he had earned during his career.
He last fought in June when he stopped Sergey Kovalev in a Las Vegas rematch to defend the titles he had won in a unification bout with the Russian seven months earlier.
Ward inflicted the only defeats of Kovalev's career and his perfect 32-fight record also boasts victories over names such as Carl Froch and Mikkel Kessler during time spent at super-middleweight.
"To the sport of boxing - I love you," wrote the 2004 Olympic gold medallist. "You've been by my side since I was 10 years old. You've taught me so much. You've humbled me. You've promoted me.
"As I walk away from the sport of boxing today, I leave at the top of your glorious mountain, which was always my vision and my dream. I did it. We did it."
After turning professional after his Olympic success in 2004, Ward took five years to land his first world title with a shock victory over Kessler.
That success arrived in the Super Six World Boxing Classic tournament, where Ward later beat Arthur Abraham and Froch to take the trophy and unify the WBC and WBA belts at super-middleweight.