The U.S. Attorney's office said Phillip A. Collier, 40, of Brooklyn, entered the plea on Tuesday afternoon.
Collier is accused of robbing the store with an accomplice back on August 6, 2012.
Police said Collier entered the store and confronted a clerk. Collier then allegedly pulled a gun and ordered four female employees to fill a bag full of jewelry.
Collier then ordered the workers into a bathroom inside the office then fled from the store, police said.
Troopers were quickly given a description of the getaway vehicle and a license plate number.
The vehicle, described as a brown Ford Expedition with a Georgia license plate, was spotted on I-95 north of Churchman's Road.
Police followed the vehicle and stopped it on I-295 just prior to the Route 9 exit in New Castle.
The jewelry, valued at $160,000, was found inside the vehicle. Police say they also found a gun that was reported stolen from Virginia Beach, Va.
Collier is also accused of robbing a Zales jewelry store in Owings Mills, Maryland on May 7, 2012.
A sentencing hearing is scheduled for September 19th.