He said he hoped to be ready for the 2012 Summer Olympics in London. Thorpe retired in November 2006 at age 24.
"Today I can say that I am now 100 percent mentally and physically committed to competing at the Olympic trials and am doing all in my power to best prepare for them and beyond," Thorpe said at a news conference.
He won an individual gold in the 400-meter freestyle at the 2000 Games in Sydney and was part of two winning relay teams at those Olympics. He won the 200- and 400-meter freestyle events in Athens four years later.
If Thorpe returns to the form he reached at the peak of his career, it would set up an anticipated duel with Michael Phelps of the United States.
Phelps won eight gold medals at the Beijing Olympics in 2008, five of them in individual events. That brought his career Olympic gold medal total to 13, most in the history of the Games.