Marseille confirm De Zerbi as new coach
The club announced earlier in the week that it had an "agreement in principle" with De Zerbi but a new statement on their website cemented the deal.
"Marseille are delighted to announce the signing of the Italian coach for the next three seasons," the statement said.
De Zerbi, 45, replaces Jean-Louis Gasset at the helm of the 1993 Champions League winners.
"I had a very strong desire to join the club," said De Zerbi.
"The history and prestige that surround Marseille, the passion and fervour of its supporters, and the seriousness and enthusiasm shown to me were all decisive factors.
"I can't wait to sit on the bench at the Velodrome, this time as OM coach, and help the club regain the position that Marseille deserve."
American owner Frank McCourt welcomed the arrival of De Zerbi: "Roberto understands the solidity of the sporting project that (president) Pablo (Longoria) is building, and I'm convinced that his passion and commitment to the club's future will be invaluable."
De Zerbi's move to Marseille is considered a coup for the southern club, who finished eighth in the table and miss out on European football next season.
The former AC Milan and Napoli midfielder had been linked with taking over at Chelsea before Enzo Maresca was appointed as their new boss.
During De Zerbi's maiden campaign at Brighton he led them to an FA Cup semi-final loss to Manchester United and to Europa League qualification for the first time.