'Rowdy' Roddy Piper -- Cause of Death ... Heart Attack from Blood Clot

"Rowdy" Roddy Piper died from a blood clot in his lungs ... TMZ has learned.

We've obtained a copy of the death certificate, which shows the wrestling legend had pulmonary embolism (a blood clot) that triggered a heart attack.

The death certificate helps explain the clot ... Roddy suffered from hypertension for years, resulting in chronic high blood pressure. Clotting can occur as a result of high blood pressure.

And the document reminds everyone how much of a force Roddy was in his profession ... his occupation is listed as "PROFESSIONAL WRESTLER ... 45 YEARS."