The straphanger was aboard the train when it pulled into the Utica Avenue station at 4:22 a.m. June 23, police said.
That's when the suspect shoved a gun in his face, according to the NYPD.
The mugger took the man's cell phone and cash before dashing off the train at the station, police said.
Cops said the suspect was last seen wearing a green shirt and beige pants.
Anyone with information is asked to call Crime Stoppers at (800) 577-TIPS (8477).