The robber was described as a dark-skinned black male, between 5 feet 5 inches and 5 feet 9 inches tall, weighing 220 to 230 pounds and wearing a white T-shirt and blue jeans, police said in a news release.
The man tried to cover his face with a bandanna during the assault, police said. Kifer described the assault as "minor."
The robber fled towards Noland Drive, police said.
Tuesday afternoon's robbery qualifies as an armed robbery because the man claimed to have a weapon, Kifer said.
It was the third armed robbery in four days in the city.
City police responded to an armed robbery at the Super 8 Motel late Friday and one on Locust Street on Saturday night, according to Kifer.
Tuesday's robber kept telling the 17-year-old ice cream store clerk that he would kill her if she didn't cooperate, the girl told a reporter Tuesday afternoon as she sat outside The Big Dipper.
All she could think about during the assault was her 4-month-old daughter, the girl said.
She worked in food service before getting a job at The Big Dipper, but had never been robbed before, she said.
The ice cream shop, which sells Hershey's ice cream and soft-serve treats, was never robbed before, owner Selina Grogg said.
She might make some changes in light of the robbery, Grogg said.
Anyone with information about "Scoop" or "Scoops" may call police at 301-790-3700, ext. 234.