The cause of death was "acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam and doxylamine," spokeswoman Ellen Borakove said in a statement.
The drugs are the generic names for the painkiller OxyContin, the anti-anxiety drugs Valium and Xanax, and the sleep aids Restoril and Unisom. Hydrocodone is a widely used prescription painkiller.
Borakove wouldn't say what concentrations of each drug were found in Ledger's blood, or whether one drug played a greater part than another in causing his death.
"What you're looking at here is the cumulative effects of these medications together," she said.
The ruling comes two weeks after the 28-year-old Australian-born actor was found dead in the bed of his rented SoHo apartment. Police found bottles of six types of prescription drugs in his bedroom and bathroom.
Ledger's family returned to the actor's hometown of Perth, Australia, on Tuesday to prepare for his funeral. Arrangements were private.
In a statement released through Ledger's publicist, the actor's father, Kim, said Wednesday: "While no medications were taken in excess, we learned today the combination of doctor-prescribed drugs proved lethal for our boy. Heath's accidental death serves as a caution to the hidden dangers of combining prescription medication, even at low dosage."
Ledger, nominated for an Oscar for his role in Brokeback Mountain, had returned to New York from London, where he had been filming a $30 million Terry Gilliam film, days before his death. He said in a November interview that his most recent completed roles in the Batman movie The Dark Knight and Bob Dylan biopic I'm Not There had taken a toll, saying he couldn't sleep.
"Last week I probably slept an average of two hours a night," Ledger told The New York Times. "I couldn't stop thinking. My body was exhausted, and my mind was still going." He said he had taken two Ambien pills, which only gave him an hour of sleep.
TV Guide reports that the federal government is getting involved in Heath Ledger's death. The Drug Enforcement Administration has launched an investigation into how Ledger obtained the multiple prescription drugs that led to his accidental overdose. The DEA has requested copies of Ledger's autopsy as well as the original police report from the crime scene, according to local law enforcement officials.
The Dark Knight was released July 18th, 2008 and stars Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine, Maggie Gyllenhaal, Gary Oldman, Morgan Freeman, Monique Gabriela Curnen. The film is directed by Christopher Nolan.