Michael Phelps, the most decorated Olympian of all time, on Monday joined the board of an Australian mental health company.

The American, who won 28 Olympic medals, quit swimming last year and he said joining Medibio met his aim to raise awareness of mental health problems.

"I personally suffered from mental health challenges from my teenage years on, and only fairly recently -- after reaching a point of desperation -- did I acquire the understanding, treatment, and support I needed, which has truly changed my life," he said.

"For me, self-awareness, from a mental health standpoint, is empowerment."

Medibio has developed a test to help diagnose depression, chronic stress and other disorders, using circadian, sleep and other information such as blood pressure.

Phelps, who was diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) when he was nine, is already an ambassador for the Child Mind Institute's #MyYoungerSelf social media campaign to help end the stigmas associated with mental health and learning disorders.

He isn't the only swimming great to suffer mental problems: Australia's Ian Thorpe has also struggled with depression, and received treatment in 2014 after being found disoriented on a Sydney street.

Fellow Australian Grant Hackett also sought help this year -- including from Phelps -- after his arrest following a family bust-up, the latest in a series of unsavoury incidents involving the troubled star.

"I want to help others who are dealing with these challenges and make the process for them to take action easier and more understandable," Phelps said as part of Medibio's announcement to the stock market.

Phelps is widely seen as the greatest swimmer ever to compete, but it came at a price.