The court in Paris found Eric Blanchemain guilty of "public insult" for the November 2010 posting on the Facebook page of the CGT-FAPT union branch for the Webhelp call centre based in Caen, northwestern France.
In the post, Blanchemain had written: "Crap day, crap weather, crap job, crap office, crap bosses."
He was given a suspended sentence of a €500 ($635) fine and ordered to pay a symbolic one euro each in damages to Webhelp, its branch in Caen and his immediate supervisor.
"The statements in question went beyond the limit of acceptable criticism," the court said in its ruling.
Blanchemain, who had been suspended for five days following the post, admitted in court to having posted the statement but said it was under exceptional circumstances as it was the day after a colleague's suicide.