AC Milan chief executive Adriano Galliani has confirmed the move for the 21-year-old.
Cissokho will sign a five-year contract with the Rossoneri.
“The negotiation during the night has been intense”, Galliani told Milan’s official website.
“We consider Cissokho really strong and this is showed by the interest towards him, also in the decisive hours of the agreement, arrived from important European clubs.
“But in the end the friendship prevailed, after many years in football, with the Porto directors, with whom in the past we were co-founders of the G-14.”