The 38-year-old former judge on NBC's "The Voice," whose real name is Thomas DeCarlo Calloway, entered the plea on the day his preliminary hearing was scheduled to begin.
Green was sentenced to three years' formal probation, 360 hours of community service and ordered to complete 52 Alcoholics Anonymous or Narcotics Anonymous meetings.
He's due back in court in March for a progress report.
Green was accused of giving a 33-year-old woman Ecstasy, a drug also known as Molly, at the Wokcano restaurant in July 2012.
A Los Angeles police investigation began shortly after the woman reported to authorities that Green put the drug in her drink as they dined together. The woman, who has not been named, told investigators she woke up to find herself naked in a bed with the singer in her hotel room.
But after two rounds of investigations by the
Green's attorney, Blair Berk, described the interaction between Green and the woman as "consensual relations."
Sources familiar with the investigation said Green allegedly acknowledged providing the woman the drug in a documented telephone call after the incident.
Before a court appearance last year, Berk said the singer was "pleased" with the prosecutors' decision not to file any sex charges.
"Cee Lo had faith that if true facts were known, the district attorney would reject those charges," Berk said of the sex assault case.