Authorities say the suspect attacked a man who tried to intervene in a dispute between two men on a southbound C train on Wednesday just before 10 p.m.
The 44-year-old victim was slashed in the face with an unknown sharp object near the Clinton-Washington Avenue subway station.
Police say the suspect then exited at the Franklin Avenue station.
The victim was taken to a local hospital and was treated for a laceration to his face.
The NYPD released surveillance video of the person they are looking for. He was described as a man with short dark hair and approximately 5'8" tall.
He was last seen wearing a baseball hat, black hoodie and dark-colored pants.
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