The robbery occurred at 10:30 a.m. at the branch on the 1200 block of Chestnut Street.
Investigators say the man entered the bank armed with a silver colored handgun and fled the bank with an undetermined amount of cash.
The suspect was described as a male wearing a dark colored ski mask, ski goggles, a dark colored hooded sweatshirt, gray sweatpants, and black sneakers.
The robbery prompted police to lock down students participating in summer activities at Emmaus High School.
Officers and K-9 units swept the area near Cedar Crest Boulevard and Lower Macungie Road for clues.
But the robber remains on the loose.
No one was hurt in the heist.
Anyone with information is asked to call Emmaus Police at 610-967-3113.