Around 30 guests were eating at the McDonald's in the canton of Schwyz at the time, a reader of Swiss tabloid Blick told the newspaper.
The television screen displayed explicit ads for chat sex websites and then even showed a soft porn film, the diner said, adding other customers must have ignored the situation out of embarrassment.
Staff were informed but clearly nothing was done, the reader said.
This is not the first time a McDonald's restaurant in Switzerland has shown pornography on a television screen. A similar 2014 incident in Zuchwil in the canton of Solothurn went global after international media outlets picked up the story.
To prevent such incidents, staff at Swiss McDonald's restaurants are instructed to only select channels such as Eurosport which do not run pornographic content.
But during the recent incident in the canton of Schwyz, the television was tuned to German sports broadcaster Sport1, which does show pornography late at night.
The incident was “highly regrettable” and staff would once again be made aware of the rules,” a McDonald's spokesperson told Blick.