The midfielder arrived in January on loan from rivals Inter and immediately settled in, even scoring some rare goals.
"Muntari will continue together with Milan for the next two years," Federico Pastorello told MilanNews.
"The renewal is a pure formality and there will be no problems. Milan will make the announcement soon. The lad was able to talk to Vice-President Adriano Galliani before the end of the season, so he was reassured.
"When I was given the mandate to sell the player, I spoke to the lad and realised he wanted to gain experience and have satisfying moments after the problems he had with Inter.
"In that sense Galliani realised the good opportunity that was falling into his lap. He thought that Sulley had quality, but also great motivation, and that made the difference."