The 24-year-old victim was walking along a footpath outside the post office on Chester Road, Little Sutton, carrying an envelop full of coins to deposit at the branch when she was confronted by the man, who was armed with a pistol.
He grabbed her by the arm, spun her around and pointed the weapon into her stomach before demanding she hand over the cash.
After she gave him the envelope the robber fled towards the alleyway by the Cheshire Building Society nearby.
Police have now launched an appeal for information over the incident, which took place sometime between 4.05pm and 4.20pm on Thursday, May 8.
A Cheshire police spokesman said the force was working tirelessly to catch the man responsible, and said officers have stepped up patrols in the area to provide reassurance to the public.