Peter Greene fatally shot himself … TMZ has learned.

The Office of the Chief Medical Examiner in New York City tells us Peter’s official cause of death is a gunshot wound of the left axilla with injury of the brachial artery … and the manner of death has been classified as an accident.

In other words, Peter accidentally shot himself in the left armpit, and the bullet damaged the artery that supplies blood to the arm, elbow, forearm, and hand.

As you know … Greene was found dead in his Manhattan apartment in mid-December after a neighbor heard music coming from his apartment all night and called the police when they knocked on the door, and no one answered. Police conducted a welfare check and found Greene deceased.

Greene rose to prominence via his villainous roles in the 1990s … particularly shining opposite Jim Carrey as mobster Dorian Tyrell in the comedy “The Mask.”

He also grabbed parts in “Pulp Fiction,” “The Usual Suspects,” “Training Day,” and “The Bounty Hunter” … just to name a few of his numerous projects.

Greene was 60.