The gang struck just after 4pm and made off with a sum of money after the incident at Bridgend.
Armed response units from both the PSNI and Gardai were alerted and are now hunting the robbers.
The scene around the post office has been sealed off as part of the ongoing investigation.
A Garda spokesman said: “A group of men entered the Post Office at Bridgend at approx 4.30pm this evening, threatened staff and made off with cash.
“There were no reported injuries and Gardai at Buncrana investigating.”
It’s just the latest robbery at Bridgend.
The same post office was raided by a gang on June 8 last year.
On that occasion the gang made off with a large sum of money.
However it’s thought tonight’s raiders only got a small sum of cash.