A teen gunned down his stepfather to stop the man from beating his mother in their Brooklyn apartment, cops said.

Devin Corbin, 17, opened fire on his stepdad around 10:40 p.m. Friday — as the 30-year-old man was fighting with his mom inside the Red Hook East Houses on Hicks Street near Mill Street, where they lived, according to police.

The man’s lifeless body was discovered in the hallway.

He was rushed to Methodist Hospital, where he was pronounced dead.

Corbin was arrested and charged with murder and criminal possession of a weapon in connection to the slaying.

His stepfather’s name was not released by Saturday evening.

Cops questioned another man in connection to the slaying, but his relationship to the family was unclear.