The attack took place at 3:05 p.m. near 80th Street and South Phillips Avenue in the city's South Chicago neighborhood.
Police said the 27-year-old female worker was delivering mail, when someone stopped her, took out a gun and then robbed her.
No arrests have been made.
RELATED: Sen. Durbin calls for renewed efforts to protect USPS mail carriers amid robberies
This comes just days after Sen. Dick Durbin called on the DOJ to take more action to protect mail carriers.
Chicago police are investigating the incident.
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