The Academy Award-winning actor's treatment is scheduled to last eight weeks.
The 65-year-old Douglas says he expects to make a full recovery. He told People magazine in a statement Monday he's "very optimistic." His publicist's assistant, Eli Barach, confirmed his condition to The Associated Press.
The actor is outside the United States, publicist Allen Burry said Monday, declining to give his location or where he would be treated.
Douglas and his wife, Catherine Zeta-Jones, purchased a house on the island of Bermuda nearly a decade ago, and Zeta-Jones said they planned to raise their two children there.
Douglas won an Oscar for "Wall Street" and appears this fall in the sequel, "Wall Street: Money Never Sleeps."