The incident took place today at about 1.30pm.
No injuries were reported.
Police say the man walked into the SingPost branch wearing a white surgical mask and demanded money from the female staff at the counter.
“The female staff immediately ran into a room for safety. The male suspect then climbed over the counter and made off with about $3,000 in cash that was placed in the drawers.”
The man then ran off in the direction of Upper Serangoon Road, along Potong Pasir Avenue 1.
Police say the man was wearing a black cap, black sweater, blue jeans, black shoes, and carrying a sling bag.
Anyone with information can call the police hotline on 1800-255-0000 or submit information online at www.police.gov.SG/iwitness