A 24-year-old man was whacked on the head with a hammer while he chatted with his girlfriend on a Bronx sidewalk Sunday afternoon, according to police.

A tool-wielding suspect snuck up behind the victim and struck him across the street from 1445 Nelson Ave. in the Highbridge neighborhood around 12:45 p.m., police said.

The injured victim was taken care of by medics on the spot minutes later.

He had a gauze bandage wrapped around the top of his head to the bottom of his chin as blood appeared to drip from the back of his head, according to photos captured at the scene.

At one point, the victim, who did not have a shirt on, lay down on a piece of clothing to receive help from first responders.

He was taken to Saint Barnabas in stable condition.

The attacker fled on foot following the assault and he has yet to be arrested, police said.

More information about the hammer attack, including what the motive might have been, was not immediately available.