It happened aboard a "C" train near Nostrand Avenue on May 31st at around 8:45 p.m.
Police say the five black men started to assault the victim with an umbrella.
They exited at Nostrand Avenue and ran away.
The victim sustained a cut to his head and was treated at Brookdale Hospital.
Anyone with information in regards to this incident is asked to call Crime stoppers at 1-800-577-TIPS (8477).
You can also submit your tips by logging onto the Crime stoppers website at WWW.NYPDCRIMESTOPPERS.COM or by texting your tips to 274637 (CRIMES) and then entering TIP577.
All calls are strictly confidential.