NEW YORK, Sept 8 (Reuters) - Mathew Martoma, a former portfolio manager at billionaire Steven A. Cohen's SAC Capital Advisors LP hedge fund, was sentenced on Monday to nine years in prison for engaging in what authorities called the most lucrative insider trading scheme in U.S. history.

U.S. District Judge Paul Gardephe in New York also ordered Martoma, 40, to forfeit $9.3 million.

Prosecutors accused Martoma of making illegal trades in pharmaceutical stocks based on tips about a clinical trial for an Alzheimer's drug. They said these trades enabled SAC to generate about $275 million of illegal gains and avoid losses.

"There was nothing accidental about Mr. Martoma's conduct or the gain realized," Gardephe said. "I cannot and will not ignore that the gain is hundreds of millions of dollars more than ever seen in an insider trading prosecution." (Reporting by Nate Raymond in New York; Editing by David Gregorio)