The teacher had just finished a lecture at a business school in Zurich when he visited adult video site Pornhub.
He was caught out after forgetting to unplug the cable linking his work computer to the overhead projector, according to Swiss newspaper 20 Minutes.
While the class was empty at the time, students watching the lecture online saw the video up on the big screen.
The prof was reportedly also caught looking for pictures of naked women with no arms and legs.
School director René Portenier explained: 'I am shocked. After the course, the teacher immediately came to me and told me he had watched porn on a school computer.
'He was very embarrassed and could not tell me why he did it.'
Chairman Rolf Blitz said the 'popular' teacher had taught at the school for 40 years but added his behaviour was 'unacceptable'.
