Barnabas Marsh, also known as Old Man Marsh, was the grandson of Captain Obed Marsh. Due to his Deep One ancestry, he is functionally immortal as he has the potential of living indefinitely, though he may still be killed through violent means. This heritage also means he will gradually mutate. He ran the gold refinery in Innsmouth. When Robert Olmstead arrived in Innsmouth his transformation had progressed to the point he was unable to shut his engorged eyes, though he still wore clothing. He was presumably killed during the government's attack on the town.
He is the great-uncle of Robert Olmstead.