It happened on Tuesday around 5:15 p.m. on a northbound C train near Jay Street in Downtown Brooklyn.
Police say the suspect entered the subway car and sat down beside the 23-year-old woman.
He then allegedly showed her the handle of a gun and told her to put her cell phone in his jacket pocket.
The woman complied and the suspect fled.
Fortunately, no one was injured.
Anyone with information in regards to this robbery is asked to call the NYPD's Crime Stoppers Hotline at 800-577-TIPS.
You can also submit your tips by logging onto the Crime Stoppers Website at WWW.NYPDCRIMESTOPPERS.COM or texting your tips to 274637(CRIMES) and then entering TIP577.