An unhinged assailant randomly whacked a straphanger with a piece of metal in a Lower Manhattan subway station this week, authorities said. 

The 52-year-old victim was on the stairwell at the Brooklyn Bridge-City Hall station around 6:20 a.m. Wednesday when the stranger approached him and struck him in the head with the metal slab, cops said. 

The victim, who suffered a cut on his head, was taken to Bellevue Hospital, where he was listed in stable condition. 

The suspect fled the station on foot, and the NYPD released a photo of him late Thursday. 

No arrests have been made.