She took to Twitter to say: "A wonderful unexpected surprise has happened. My husband, Matt Cooper and I are bringing a Mini Cooper into the World.
"True to form, nothing ever works out the way we plan it. Defending my title will have to wait another four years. #pregnancytopodium
"Can’t wait to be a Mum!
"My priorities will change but the challenge of pregnancy to podium is a powerful dream.
"Very excited for a life changing 2020!"
According to the ABC, Australian Olympic Committee president John Coates also congratulated Chloe on her announcement.
"While disappointed our Tokyo team will be missing one of its brightest stars, we are delighted with her news," he said in a statement.
"Chloe has been a wonderful ambassador for her sport, Australia and the Olympic movement.
"I am sure we will see her back in Australian colours to recapture the title she won in Rio."