The victims, ages 47 and 80, were both stabbed multiple times in the upper body. Both died a short time later at a hospital.
Sgt. Monica McMenamin, an Atlantic City police spokeswoman, said Antoinette Pelzer, 44, was arrested around 10 a.m., just moments after the robbery occurred.
Pelzer initially was charged with two counts of aggravated assault and weapons offences. But Atlantic County Prosecutor Ted Housel said she would likely face more serious charges following the victims' deaths.
Pelzer remained jailed without bail early Monday night, and it was not known if she had retained a lawyer.
Housel said a city police officer who happened to be patrolling in the area witnessed the robbery and stabbing and quickly subdued Pelzer at gunpoint on a block not far from the city's casino district.
A motive for the robbery or why Pelzer stabbed the women was not immediately known, and both police and Housel declined further comment.
Investigators were working Monday night to determine how long the victims had been in the Atlantic City area and where they had been staying.
Pelzer's hometown was not immediately disclosed, but authorities said she may have recently moved to the Atlantic City area from Pennsylvania.