Philip Seymour Hoffman, perhaps the most ambitious and widely admired American actor of his generation, who gave three-dimensional nuance to a wide range of sidekicks, villains and leading men on screen and embraced some of the theater’s most burdensome roles on Broadway, died on Sunday at an apartment in Greenwich Village he was renting as an office. He was 46.

The death, from an apparent drug overdose, was confirmed by the police. Mr. Hoffman was found in the apartment by a friend who had become concerned after being unable to reach him. Investigators found a syringe in his arm and, nearby, an envelope containing what appeared to be heroin.

Mr. Hoffman was long known to struggle with addiction. In 2006, he said in an interview with “60 Minutes” that he had given up drugs and alcohol many years earlier, when he was 22. Last year he checked into a rehabilitation program for about 10 days after a reliance on prescription pills resulted in his briefly turning again to heroin.

“I saw him last week, and he was clean and sober, his old self,” said David Bar Katz, a playwright, and the friend who found Mr. Hoffman and called 911. “I really thought this chapter was over.”