The robbery happened just before 1:00 p.m. at the TD Bank at 502 Ridge Pike in Conshohocken.
The suspect entered the bank and gave the teller a note demanding money, police say.
While fleeing the bank, police say a red dye pack exploded, causing the man to drop some of the money.
The suspect is described as a black male, approximately 6'0" tall, heavy set with short dark hair.
The suspect was wearing a dark blue hooded sweatshirt and dark-colored cargo pants.
Anyone with information is asked to contact the Plymouth Township Police Department 610-279-1901 or email email@example.com.