A man cold-cocked a straphanger inside a Manhattan subway station — leaving him with a cracked skull, police said Thursday.

The crazed man was on the platform in the 34th Street station near Penn Station at about 12:30 p.m. on Sunday when he began arguing with a 28-year-old man, then punched him in the face, officials said.

The victim fell to the ground and hit his head on a nearby handrail.

When the man woke, his attacker had fled, police said.

He took himself to Good Samaritan Hospital in West Islip, where doctors admitted him with a cracked skull and swelling, authorities said.

Police released surveillance photos of the straphanger suspected in the brazen beating Thursday, showing him in a denim jacket and with a beard.