'Alaskan Bush People' Matt Brown Cause of Death Revealed

Matt Brown's autopsy is shedding a little light on his tragic death. According to the Okanogan County Coroner, the "Alaskan Bush People" star was high on meth when he took his own life, TMZ has learned.

The coroner's office tells us Matt died by suicide, and the official cause of death is a "single penetrating gunshot wound to the head."

The autopsy also says he was under the influence of methamphetamine ... stating "subsequent immersion in water (river)" contributed to his death. The coroner's office plans to release Matt's body to his family for final arrangements.

As we previously reported, his family feared the worst when the Okanogan County Sheriff's Office started searching for an unnamed man who was last seen face-down in the Okanogan River, getting swept away by the current.

At the time, Matt's brother, Bear, was told a witness spotted MB near a river, floating in the water at some point.

After a days-long search, a private search party found Matt's body. As we previously reported, his family's torn about the sad news ... feeling shock, grief and relief all at once.

Matt had been estranged from his family for a few years due to erratic behavior, but the family said in their tribute that they never lost hope that he could "heal and find peace."

RIP