YouTube star Adam the Woo died from natural causes, TMZ has learned.

A rep for Florida's Orange County Medical Examiner tells TMZ ... Adam died of "atherosclerotic and hypertensive cardiovascular disease with obesity."

Adam's dad, Jim Williams, reached out to fans in a Facebook post Monday, writing, "Our beloved Son . . . your beloved friend . . . died, in essence, of a heart attack in his sleep from health issues he probably never knew he had."

He continued ... "Now, we can all stop guessing. Be grateful the Lord allowed him to die at home and not in a foreign country. Be grateful he was found by friends (as hard as that was) and not some nameless stranger."

As we reported, Adam died December 22. Deputies initially went to his Orlando, Florida home for a welfare check, but found nothing out of the ordinary.

A concerned friend later borrowed a ladder and looked through a third-story window -- spotting Adam lying motionless on a bed. Deputies returned to the home and pronounced him deceased at the scene.

Adam rose to fame through his wildly popular YouTube channel, where he chronicled travel adventures, theme parks, roadside oddities, and pop-culture landmarks -- building a massive following along the way.

He was 51.

RIP