The 70-year-old "Love Story" and "Paper Moon" star successfully battled leukemia in the late 1990s.
"Recently, I was diagnosed with stage 4 prostate cancer," O'Neal said in a statement released by his publicist exclusively to People.com. "Although I was shocked and stunned by the news, I feel fortunate that it was detected early and according to my extraordinary team of doctors the prognosis is positive for a full recovery.
"I am deeply grateful for the support of my friends and family during this time, and I urge everyone to get regular checkups, as early detection is the best defense against this horrible disease that has afflicted so many."
O'Neal's longtime girlfriend, actress Farrah Fawcett, died of anal cancer in 2009.