The issue follows the claims made against Hollywood film producer Harvey Weinstein, which has led to many in other industries speaking out about their own experiences.
In an email to staff, Yannick Bolloré, Havas' chief executive, said: "In recent weeks, we’ve all seen several widely-publicised sexual harassment charges involving notable companies and individuals.
"These events have prompted a renewed discussion on this extremely important topic. I want to reiterate that we at Havas Group simply do not tolerate harassment – in any form. Havas has a culture that is built on ethical conduct and respect for each other."
He added that harassment can be physical and verbal, and neither is acceptable.