The 37-year-old Team Ineos Grenadiers rider, who turned professional aged 24, said on social media that it had been "a brilliant ride".
"I could never have imagined as a young kid growing up in Tasmania I'd be lucky enough to travel the world riding a bike, ride with some of the best teams in the cycling world and meet so many brilliant people along the way," he wrote.
"I'm very much ready to enjoy the next chapter but what a brilliant ride it was.
"As Forrest Gump said ‘I’m pretty tired. I think I’ll go home now’."
Porte's long list of achievements also included a third-place finish behind winner Tadej Pogacar in the Tour de France two years ago.
He also won the Tour de Romandie, Tour de Suisse and the Criterium du Dauphine, among others, but was also frequently hit by illness and injuries.
