Davy Jones died from a heart attack ... this according to the Martin County Medical Examiner.
TMZ has learned ... an autopsy was performed at 10:30 AM EST, and doctors determined Jones died from an abnormal heart rhythm caused by coronary artery atherosclerosis -- in other words, a severe heart attack.
The M.E. still has toxicology results pending, but it was obvious from the autopsy that the heart attack killed the lead singer of the Monkees.
TMZ broke the story ... Jones died yesterday after complaining of shortness of breath.