Nkem Ndefo shared the tragic news Saturday ... saying she was heartbroken at the death of her younger brother -- and hoped he'd finally found peace. She did not share a cause of death.

Obi got his start in Hollywood back in the mid-'90s appearing in bit roles on shows like "Star Trek: Voyager," "3rd Rock From the Sun" and "Angel" ... before getting his big break on "Dawson's Creek."

Ndefo appeared in 10 episodes of 'DC,' playing Bodie Wells during its six-season run -- dating Nina Repeta's Bessie Potter during his time on the show.

Mary-Margaret Humes -- who played Dawson's mom on the show -- posted a loving tribute to him after news of his death broke ... calling him a bright, shining light in spite of all the adversity he faced.

Among that adversity, Ndefo lost both his legs in 2019 when a driver hit him while he was putting groceries in his car, completely severing one leg and requiring the other to be amputated.

Obi used his own experiences to try and help others ... creating a podcast all about battling through major trauma like the kind he faced.

Obi was 51.