The incident happened in front of the customers during a late-night argument at a Pita Pit restaurant in Montana.
The woman who shot the video says she and her family went into the restaurant around 2 a.m. and the employee was rude as soon as they walked in.
Video: restaurant worker spits in customer's food
In the video, you can hear a customer dare the employee to spit in the food.
She takes the dare and spits in the sandwich. A customer then jumps over the counter and tries to grab it and the employee throws it over the counter.
They exchange angry words and the employee indicates she's not too happy about still working there late into the night.
The original video can be viewed here. Warning: contains some graphic language. It has been viewed nearly 500,000 times since it was posted over the weekend.
The owners of the Pita Pit in Missoula issued an apology on Facebook and said the employee had been fired. The husband and wife owners of the restaurant said they were "mortified" at the employee's behavior.
"I can see what happened during the video and there is no excuse whatsoever for the type of behavior shown by an employee that I can assure you is no longer working for us," they wrote.