It happened Saturday night about 10 o'clock, but it wasn't until today that Sioux City Police released videotape of the incident.
The robbery took place at the Kum & Go store in the 2600 block of Court Street.
You can see two men walk into the store. One of them uses the bathroom, then they both leave.
Moments later one of them acts as a lookout, while the other comes back into the store with a knife and robs it.
The video is posted on the Sioux City Police Department's Facebook page.
If you have any information about the robbers, please contact crime stoppers at 712-258-TIPS.
Your identity is not required and a cash reward is available.