Investigators say it happened at about 10:45 a.m. on March 30th on the southbound platform of the R train at the 4th Avenue-9th Street subway station in Park Slope.
The suspect approached the woman, struck her in the head with his fist, and fled on foot.
The NYPD has released a surveillance photo of the suspect.
Anyone with information is asked to call Crime Stoppers at (800) 577-TIPS (8477).
The public can also submit their tips by logging onto Crime Stoppers' website at http://www.nypdcrimestoppers.com or by texting their tips to CRIMES (274637), then enter TIP577.