A good Samaritan was slashed across the face inside a Brooklyn train station this week when he tried to stop a man who had slapped another straphanger, cops said. 

The suspect was harassing other people inside the Bedford Avenue L train station in Williamsburg around 10:20 p.m. Sunday, police said. 

When the assailant slapped another commuter in the face, the 36-year-old do-gooder tried to intervene, authorities said. 

Then the attacker turned on him and slashed him across the face with a “cutting instrument,” cops said. 

The victim was taken to Bellevue Hospital, where he was listed in stable condition. 

Police released a photo of the suspect, seen walking with a bike, late Monday.