The robbery occurred at an apartment complex at the 1300 block of 13th Street and Northwest 43rd Avenue, a little before 12:30 p.m., Wednesday.
According to the US Postal Inspection Service, a female mail carrier was delivering mail to the apartment complex when an armed man approached her and demanded her mailbox key and cell phone.
She gave them to the robber, and he fled, leaving her unharmed.
The subject is described as standing between 5 feet 6 inches and 5 feet 8 inches.
He was last seen wearing a black cap with a red bandanna around his face.
The USPS will re-key the mailboxes in the area to render the stolen key useless.