The charitable actor returned to his native Maine to take part in the cycle challenge fundraiser in aid of the Central Maine Medical Center, where he was born and where his mum recently endured three courses of cancer treatment.
Dempsey, who has set up his own Patrick Dempsey Center for Cancer Hope & Healing at the hospital, organised the event and joined 3,500 other athletes to take part in the Dempsey Challenge - a five mile (five kilometre) road race and 50 mile (80 kilometre) bike ride.
The event raised over $1 million (£666,000) for the cause and Dempsey admits he is delighted because the hospital will always mean a lot to him.
He says, "I learned a lot during my mother's illness. I found out how important it is to have access to good, reliable resources to help the patient, family members and caretakers (and) I knew I that I wanted to make a difference."