The Ultimate Warrior died from a massive heart attack ... this according to the medical examiner who conducted the autopsy, TMZ has learned. 

Officials tell TMZ ... the official cause of death for Warrior -- real name James Hellwig -- was determined to be "Atherosclerotic/Arteriosclerotic Cardiovascular Disease" ... aka heart disease. 

Officials classified Warrior's death as "natural" -- meaning neither drugs nor alcohol were directly involved.

As we previously reported, Warrior clutched his chest outside of an Arizona hotel on April 8 while walking with his wife. He was rushed to a nearby hospital, but never regained consciousness. 

Warrior was 54 years old.