Metro Police said the unidentified man walked into the Dairy Queen with a samurai-style sword about 12:19 p.m. The employee apparently fired at him twice, hitting him in the chest, KLAS-TV, Las Vegas, reported.
The alleged robber was pronounced dead at Sunrise Hospital, the TV station said.
The Las Vegas Sun reported police Lt. Leslie Lane said he didn't think the employee, whose name wasn't reported, would be charged.
Kimberly Phillips-Atkinson, who works in a nearby store, told the newspaper she went outside after the shooting and saw two young men who work at the Dairy Queen sitting on a curb crying.
"If that guy thought he could get away with this ...," she said. "Now he's dead. It's very sad."