A Brooklyn woman was beaten and robbed — but a good Samaritan found her bag, which was missing just $5, police said Wednesday.

The 53-year-old victim was walking up the stairs of the Seventh Avenue station in Park Slope about 7 p.m. Tuesday when a woman approached her from behind, punched her in the face and knocked her to the ground, cops said.

The assailant, believed to be in her 30s, grabbed the victim’s bag and fled into the subway station.

Moments later, a good Samaritan retrieved the bag and returned it to her with only $5 missing, police said.

The victim refused medical attention at the scene.