Former European champions Marseille revealed earlier this week an "agreement in principle" had been reached for the former Brighton boss and it has now been made official.
De Zerbi left the Seagulls at the conclusion of the 2023/24 campaign after a successful 20 months at the Premier League club, who he guided into the Europa League for the first time.
"I'm delighted to be joining Olympique de Marseille," De Zerbi, 45, said.
"The history and prestige that surround OM, the passion and fervour of its supporters, and the seriousness and enthusiasm shown to me by (club officials) were all decisive factors in my decision to embark on this exciting challenge.
"I can't wait to sit on the bench .. as OM coach and help the club regain the position that Marseille deserves."
Marseille - nine times champions of France - endured a turbulent campaign last season, finishing eighth in Ligue 1 after rotating through three permanent head coaches.
"We are very proud to welcome Roberto to Olympique de Marseille, a talented coach with a European reputation," Marseille president Pablo Longoria said.
"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."
