The landmark case involved comments made about Erwin Kessler, the head of an animal protection group.
He was accused of being anti-Semitic and racist, media reports say.
The Zurich district court said the defendant "clearly endorsed the unseemly content and made it his own" by liking comments.
The 45-year-old man liked six comments, according to Swiss newspaper Le Temps.
Mr Kessler is reported to have sued more than a dozen people over various comments made on Facebook in 2015.
They were made in relation to a discussion over which animal rights groups should be allowed to participate in a vegan festival, the Tages Anzeiger newspaper reports.
Several people have been convicted but none for only "liking" comments made by others.
According to the AFP news agency, Mr Kessler was convicted under anti-racism laws nearly 20 years ago.
The Zurich court ruled that the defendant had not been able to prove that the comments he liked were truthful.
It also said that the action of "liking" the comments "made them accessibly to a large number of people", and were thus an "affront to [Kessler's] honour".
He was given a conditional fine of 4,000 Swiss francs (£3,200; $4,100), according to AFP.
The verdict can be appealed against.
A lawyer for one of the other people sued by Mr Kessler said the ruling could "have a large impact" despite being from a regional court.
Amr Abdelaziz said the Swiss courts needed to offer clarity to social media users and warned freedom of expression could be jeopardised if courts want to prosecute people over Facebook "likes".