Screenwriter David Katz and another friend called 911 after they went to check on the actor and found him unconscious on his bathroom floor around 11:30 a.m.
The cause of death has not yet been determined, but according to NYPD sources, glassine envelopes containing heroin was found at the scene. Law enforcement officials told ABC News that a hypodermic needle was sticking out of Hoffman's arm.
The versatile actor struggled with drug abuse early in his career and had previously been in rehab for drug and alcohol addiction. After more than 20 years of sobriety, Hoffman admitted last year that he relapsed and did another stint in rehab after using heroin.
Hoffman won an Oscar for his leading role in the 2005 film "Capote". He has also been nominated for his performances in the movies "The Master," "Charlie Wilson's War" and "Doubt" and has also appeared in films such as "The Big Lebowski" and "Almost Famous" and "The Hunger Games."
He starred in "The Hunger Games: Catching Fire," playing Plutarch Heavensbee.
Hoffman is survived by his longtime girlfriend, costume designer Mimi O'Donnell, and their three young children.
Hoffman's family released the following statement Sunday: "We are devastated by the loss of our beloved Phil and appreciate the outpouring of love and support we have received from everyone. This is a tragic and sudden loss and we ask that you respect our privacy during this time of grieving. Please keep Phil in your thoughts and prayers."
Police will only say the investigation is continuing. An autopsy is planned for Monday, according to medical examiner spokeswoman Julie Bolcer.
ABC News and The Associated Press contributed to this report.