Douglas has co-starred with his father Michael and grandfather Kirk
The son of Oscar-winning actor Michael Douglas admitted in a New York court to possessing heroin and dealing large amounts of methamphetamine and cocaine.
Cameron Douglas, 31, was arrested last July at a hotel in Manhattan. If convicted he faces a minimum 10-year prison sentence.
When asked by US District Judge Ronald Ellis if he knew what he was doing was wrong, he said: "Yes, your honour,"
Prosecutors said he has signed a plea agreement, but gave no further details.
In addition to conspiracy to distribute drugs, Douglas pleaded guilty to heroin possession.
The charge stems from allegations that his girlfriend tried to smuggle him the drug in an electric toothbrush while he was under house arrest.
Cameron Douglas appeared with his father and grandfather, actor Kirk Douglas, in the film It Runs in the Family in 2003, a comedy focusing on three generations of a dysfunctional family.
His mother is Diandra Douglas, who was married to Michael from 1977 to 2000.
Douglas is currently married to actress Catherine Zeta Jones.