Ledger was found naked and unresponsive, facedown on the floor at the foot of his bed by a housekeeper trying to wake him for an appointment with a masseuse, said police spokesman Paul Browne.
"Pills were found in the vicinity of the bed," he told CNN.
"This is being looked at as a possible overdose, but that is not confirmed yet."
Browne later told reporters some prescription medications were found in the room, including sleeping pills.
But he stressed police have made no determination of the cause of Ledger's death -- that would be done by the medical examiner.
He said the pills were not "scattered about."
No note was found and there was no indication of foul play, Browne said. Ledger was found at about 3 p.m., and was pronounced dead at the scene by emergency personnel about 3:30 p.m.
A crowd of onlookers, photographers and reporters gathered outside the apartment building after news of Ledger's death was reported. Police officers were guarding the doors.
Browne said he did not know how long Ledger had been renting the apartment, which he said took up the entire fourth floor.
An autopsy would be conducted on Wednesday, said Ellen Borakove, spokeswoman for the New York City medical examiner's office.
Ledger's family called his death "very tragic, untimely and accidental."

"Heath has touched so many people on so many different levels during his short life, but few had the pleasure of truly knowing him," his father said.