The Italian has spoken for the first time since his appointment announced.
De Zerbi said, "I'm delighted to be joining Olympique de Marseille, as I had a very strong desire to join the club.
"The history and prestige that surround OM, the passion and fervour of its supporters, and the seriousness and enthusiasm shown to me by Frank, Pablo and Medhi were all decisive factors in my decision to embark on this exciting challenge.
"I can't wait to sit on the bench at the Orange Vélodrome, this time as OM coach, and help the club regain the position that Marseille deserves."
OM co-owner Pablo Longoria also remarked: "We are very proud to welcome Roberto to Olympique de Marseille, a talented coach with a European reputation. As well as being enthusiastic about the idea of coaching OM from the outset, he has all the qualities required to succeed on our bench.
"His decision to join us sends out a strong signal about the club and the new sporting cycle we are embarking on."