Nicholas Brendon's friend found him dead ... and authorities' initial assessment is that his death appears to be from natural causes.

According to Putnam County Coroner Todd Zeiner in Indiana ... he was notified of the deceased individual Monday -- and, when he arrived, he found "an adult male positioned as if asleep" who had already been pronounced dead.

While Zeiner says Brendon lived alone, a longtime friend -- who had been staying at the house overnight to provide care -- found that he had died. The time of death was shortly before the body was found, the coroner says ... unclear what time of day that was.

Brendon's family members reported a history of cardiac issues -- he suffered a heart attack in 2023 and was later diagnosed with a congenital heart defect -- and the coroner says there were "no signs of foul play, and nothing at the scene indicated the death was anything other than natural."

The autopsy and toxicology results are still pending.

Brendon's family announced his death in a statement to The Hollywood Reporter earlier this week ... stating he "passed in his sleep of natural causes."

Brendon had a history of legal issues ... including being charged with domestic violence for an incident in 2017 where he allegedly attacked his girlfriend.