Benji Gregory -- best known as the child star on the sitcom "Alf" -- has passed away ... TMZ can reveal.

The actor's sister Rebecca tells TMZ her brother was found dead in his car at a Chase Bank parking lot in Peoria, AZ, on June 13.

His death certificate confirms that date, and the Maricopa County Medical Examiner's Office tells us Benji's cause of death is still pending.

Rebecca tells us her brother suffered from depression, bipolar disorder and had a sleep disorder that often kept him awake for days.

Those close to him believe he went to the bank to deposit residual checks on the 12th, fell asleep in his car and tragically died from vehicular heatstroke due to Arizona's intense summer heat.

Benji was with his service dog Hans, and the animal also died.

Rebecca tells us their family is grieving a great loss and appreciates everyone's thoughts and kind words. She suggests donations in Benji's name would be appreciated to either The Actors Equity Foundation or the ASPCA -- causes he would've supported.

Benji skyrocketed to fame playing the role of Brian Tanner on 101 episodes of "Alf," which aired from '86 to '90.

He had a few other acting gigs here and there, but shifted out of entertainment in 2003 when he enlisted in the U.S. Navy ... later graduating from school to become an aerographer's mate. In 2005, he received an honorable medical discharge from the Navy.

Benji was 46.