The suspect approached Richianand Brijanand, 32, whose name has not been released, at approximately 9:20 a.m. inside the Rockaway Avenue C train station at Fulton Street after he purchased a MetroCard and hit him in the head with a mallet and demanded money.
The thief, who can be seen grinning in the surveillance video, ran off after the man handed over the cash.
The victim was taken to Brookdale hospital where he was treated for a head injury, police said.
The suspect is said to be 5-foot-10 and weighing between 130 and 150 pounds. He is seen wearing a red sweater in the video.
Police are asking anyone with information to contact Crimestoppers at 800-577-TIPS (8477).