An unhinged assailant randomly struck a straphanger in the back of the head with a hammer as he waited for a Manhattan train, cops said early Tuesday. 

The man was on the southbound A train platform at the 175th Street station in Washington Heights at 11 a.m. Sunday when a stranger came up from behind and attacked, authorities said. 

“I’m gonna f–k someone up,” the suspect declared, according to cops. 

The victim, 46, went to the booth clerk for help, cops said. He was treated on scene by EMS for head pain but refused further medical attention.

The suspect took off and remained at large Tuesday. 

A photo released by cops shows him entering the station before the assault.