The American won a re-match with Sergey Kovalev by technical knockout in the eighth round in Las Vegas in June, having defeated the Russian in a unanimous points decision in November 2016 to unify the WBA, IBF and WBO belts.
There had been talk of Ward fighting David Haye's recent conqueror Tony Bellew at cruiserweight or even jumping up the divisions to take on heavyweight champion Anthony Joshua.
A light heavyweight gold medal winner at the 2004 Olympic Games in Athens, Ward calls time on his undefeated career with a record of 32 victories, 16 by knockout.
He previously held the unified WBA and WBC super middleweight titles following landmark victories over Mikkel Kessler, Carl Froch and Chad Dawson.
"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.
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