A thug mercilessly beat a 36-year-old man with a baseball bat in broad daylight yesterday in Brooklyn, cops said.

The horrific assault occurred at about 12:50 p.m. on 20th Avenue near 78th Street in Bensonhurst.

The victim, whose name was withheld, suffered serious head trauma and was listed in stable condition last night at Lutheran Medical Center.

A 911 caller described the assailant as wearing a black jacket.

It was not immediately known whether the victim knew his attacker or whether anything was taken from him.