It happened on Monday at about 1 p.m. in the vicinity of East 21 Street and Avenue U in Sheepshead Bay.
Polce say two males approached the 24 year-old man and demanded his property.
When the victim refused, one of the robbers displayed a handgun and snatched the victim's backpack off his shoulder, police say.
The suspects then fled eastbound on East 21 Street with approximately $80,000 in cash.
Upon further investigation, police determined that two other individuals, a male and a female, acted as lookouts throughout the incident.
One of the suspects who robbed the victim exited a black four door sedan prior to the incident.
Anyone with information is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).