The Office of Chief Medical Examiner released its findings Friday. Its report said Hoffman died of "acute mixed drug intoxication."
The Oscar-winning actor was 46 when he died Feb. 2, alone on the floor of the bathroom of his Manhattan apartment.
Police said a needle was found in his arm and bags of heroin were scattered around his apartment.
The New York Post said the type of drug cocktail Hoffman ingested is called a "speedball."
Hoffman was clean and sober for 20 years after beating an addiction to drugs as a college student but he relapsed and began taking heroin and prescription drugs in 2012, the Post said.