The company, headquartered in Glattbrugg in the canon of Zurich, acknowledged in a brief statement onThursday that it and some of its French competitors were being probed for "alleged violations of French competition law."
Adecco did not name the other companies under investigation but said that it was "fully cooperating with the authority."
France constitutes Adecco's main market, with 6,000 permanent staff and 140,000 temps, making it the country's largest private employer.
The Swiss company suffered from the eurozone crisis, seeing its net profit plunge 40 percent during the first quarter of this year to €67 million ($87.4 million), after seeing 27 percent of its net profit evaporate in 2012.
In France alone, Adecco's sales fell 17 percent during the first quarter, to €1.05 billion.