The incident happened just before 9 p.m. on October 18 in the vicinity of Wallace and Lydig avenues in the Bronxdale section.
Authorities say the suspect followed the 65-year-old victim into an elevator inside his apartment building.
He allegedly waited until the victim exited the elevator of the sixth floor, then punched him from behind, knocked him to the ground and removed his wallet containing an undetermined amount of cash before fleeing down a staircase.
The victim was taken to a local hospital with a laceration to his head.
The suspect is described as a Hispanic male in his early to mid 20s. He is approximately 6-feet tall and was last seen wearing a red hooded sweatshirt, dark pants and red and black sneakers.
Anyone with information with regard to this incident is asked to call Crime Stoppers at 1-800-577-TIPS (8477).