Strauss-Kahn remained impassive as the verdict was read at the end of a long trial which saw 14 people prosecuted in a prostitution-ring scandal.
The scandal, along with accusations of sexual assault on a New York chambermaid, has brought the 66-year-old's sex life to widespread public attention.
Strauss-Kahn resigned as IMF director and dropped out of the race to be the Socialist Party's presidential candidate in France's 2012 election after the New York allegations.
The Lille case, known as the "Carlton affair" after the hotel where some of the sex parties took place, revealed that he and his associates had held orgies in France, Belgium and the US.
But Strauss-Kahn denied knowing that the women involved were prostitutes and during the trial the public prosecutor called for the charges to be dismissed because they were so difficult to prove.
The judgement found that he may have known that the women were sex workers but that he only acted as a client, which is not illegal in France.
Almost all the other defendants, including notorious pimp Didier Alderwired, nicknamed "Dodo la Saumure", were also acquitted.
But former Carlton employee René Kojfer was sentenced to a one-year suspended sentence.
Three other defendants, who were acquitted of the pimping charges, were given suspended prison sentences for financial crimes.