The 22-year-old has been linked with the likes of Chelsea, Inter and Southampton in recent weeks, but put pen-to-paper on a five-year deal with the Portuguese side.
Porto confirmed the deal had been completed on Wednesday and revealed they had inserted a €50m buyout clause in his contract to deter other clubs in the future.
Imbula insists his decision to join the Primeira Liga runners up is rooted in sporting reasons and he hopes he can help to bring success to the Estadio do Dragao.
"I am delighted to have signed for Porto," the former France Under-21 international told his new club's official website. "The sporting project here seduced me.
"This is a club who have won many European trophies in their history, they have a great team and hopefully it will be even better with my contribution.
"I talked to the coach Julen Lopetegui two or three times and he convinced me to come. He told me who the team's style of play suits my qualities. I'm so glad to be here and I want to win many titles here.
"Porto is not known to throw money at young players so no one can say I came here because of money. I came here, instead, because of the trust placed in me by the coach and the club.
"Leaving Marseille to come here feels like a step up in my career and hopefully I can fulfil all the expectations Porto have about my future."