The 22-year-old Frenchman is on a season-long loan move at the Stade Velodrome.
Guendouzi has agreed a three-year contract worth a reported £9m.
Guendouzi joined Arsenal from Lorient in the summer of 2018 and made 82 appearances in total for the Gunners before falling out of favour under Mikel Arteta towards the end of the 2019/2020 season.
He has made 38 appearances across all competitions for Marseille this season, scoring four goals and providing eight assists.
“It was a carefully considered choice,” he told RMC.
“There were very good discussions with the president, with the coach and people in my family.
“We really made the best decision all together. For me, it’s the best choice I’ve made in many years.
“I’m very happy and very fulfilled. President [Pablo] Longoria and coach [Jorge] Sampaoli are the people who presented the project to me and which made me want to come.
“We talked about the OM project, what I could bring to the team, how I was going to be able to grow. I found myself in these words, I knew that by coming here I was going to cross levels and continue to grow, whether humanly or footballingly.”
