'Party Down South' Cast Member Joshua Tharpe Dead at 42

Joshua Tharpe -- a guest star on "Party Down South" -- has died ... TMZ confirms.

His brother, Billy, tells us ... Josh was found dead Monday by his mother on the kitchen floor of his home in Baton Rouge, LA. Billy says Josh's death comes as a surprise, as everything seemed fine just a day or so prior.

We're told Josh had gone to church with his mother on Sunday and then hung out at her house afterward. Later that day, he swung by a friend's place where he had plans to spend the night -- but had to stop by at his own place first to grab some clothes ... and Billy says he never returned.

The friend grew concerned when she didn't hear from him the next day, and contacted his mom ... who went to his house where she found him collapsed. Billy says the coroner has notified them that Josh died as a result of a heart attack. Apparently, he'd experienced something similar a year prior, but much more mild and he'd recovered.

Joshua wasn't a full-time cast member on the CMT reality show, but he was featured in multiple episodes.

He was just 42.