Junior Seau Autopsy -- No Drugs, No Alcohol in System

Junior Seau was completely sober when he shot himself in the chest back in May ... and his cause of death has been ruled as a suicide ... this according to the San Diego medical examiner.

TMZ obtained the autopsy report, filed by Deputy Medical Examiner Craig Nelson ... which shows Seau died of a self-inflicted gunshot wound to the chest from a .357 caliber revolver on May 2, 2012.

The report also shows "No alcohol, common drugs of abuse, or other medications were detected."

However, a small amount of Zolpidem (aka ambien) and Naproxen (an anti-inflammatory) were detected ... but the M.E. says test results were "consistent with therapeutic use."

The report notes that Seau's brain tissue was sent to the National Institutes of Health at the request of his family.

According to the report, Junior did not leave a suicide note.