Thorpe will finally end speculation about his sexuality in the tell-all interview to be broadcast on Channel 10, the Sunday Telegraph said.
In his 2012 autobiography, "This Is Me," Thorpe said, "For the record, I am not gay and all of my sexual experiences have been straight. I'm attracted to women, I love children, and aspire to have a family one day."
Thorpe, 31, retired from swimming in 2012 after winning five Olympic gold medals, three silvers, and one bronze, and setting 22 world records.
The Sunday Telegraph did not say how it knew the content of the interview but reported Thorpe "confirms his sexuality" and "bravely reveals he is gay."
A promotional clip from the interview, broadcast on Saturday, shows Parkinson asking Thorpe, "You've always said that you're not gay, is all of that true?" Thorpe's response is not shown but he is seen considering his answer.
In a statement released by Channel 10, Parkinson describes the interview as one of his best. He says Thorpe speaks frankly about his battle with depression, drugs and alcohol.
"His fight against depression offers a unique insight into the darker side of celebrity and success," Parkinson says. "Ian Thorpe has always been near the top of my list to interview. The reasons are obvious. Not many athletes can claim to be the best of all time. Ian can."
Thorpe was 14 when he was first chosen for Australa, and became swimming's youngest world champion at that age when he won the 400-meter freestyle at the 1998 worlds in Perth.
His career peaked at the 2000 Sydney Olympics at which he won three gold and two silver medals. He retired after the 2004 Athens Olympics, citing a lack of motivation, but made an unsuccessful comeback when he tried to qualify for the 2012 London Games.