Prescription drugs were found near Heath Ledger's body
The Brokeback Mountain actor, 28, was found dead in January in an apartment he had been renting in Manhattan.
The city medical examiner's spokeswoman said Ledger died "as the result of acute intoxication by the combined effects" of the different drugs.
They include painkiller Oxycontin and anti-anxiety drugs Valium and Xanax.
Some of the prescription drugs were found near his body when it was discovered by his housekeeper.
Ledger's body has been flown home to Australia for a private burial, while a public memorial service is planned in Los Angeles.
Last week, his former partner Michelle Williams spoke publicly for the first time since the star's death, saying: "My heart is broken."
The pair met on the set of their Brokeback Mountain, in which Williams played Ledger's wife.
The couple, who had a daughter Matilda, now two, split up last September.
The makers of Ledger's most recent film, The Imaginarium of Doctor Parnassus - directed by Terry Gilliam - have suspended production.
Ledger had completed filming in London just before his death.
The star also plays The Joker in yet-to-be-released Batman film The Dark Knight.
Also last week, US TV shows Entertainment Tonight and The Insider pulled a video they had planned to show of Ledger at a Hollywood party saying he used to smoke cannabis.
The star is apparently heard saying he "used to smoke five joints a day for 20 years".The shows released a statement saying the footage, believed to be have been recorded in January 2006, would not be shown.