The 23-year-old was on a Red Line train around 12:45 a.m. in the 800 block of North State Street when two people approached, showing a handgun and demanding his property, police said.
They took his cellphone before exiting the train, police said. Officers met with the victim at the Belmont Red Line station.
No one was injured during the robbery, police said.
No one was in custody as Area Three detectives investigate.