The 24-year-old, a Germany international who mainly plays in the middle of the back, will be the third addition of the third team of the summer of summer.
Malick collected 85 performances in all competitions during his three -year enchantment with the Rossoneri, including a against Eddie How's side in the San Siro in the UEFA Champions League.
His only goal for Milan arrived in the European Elite competition last season and found the target in a 3-1 victory to Real Madrid.
Malick started his professional career at Homeland-Kant Schalke 04, where he progressed through the youth system before he made his Bundesliga debut as an 18-year-old in 2020. The defender has three Germany caps in his name at an international level.
This season, Malick will wear shirt number 12 for Newcastle United.
Malick said: “I am very enthusiastic to become a member of this huge club. I can't wait to start training and to get to know my new teammates and all the staff.
“The Gaffer showed me his vision and what he wants to do with me as a player and the club himself, which is really exciting.
“I think Newcastle speaks for himself. It's a great club, a big club with passionate fans that I can't wait to see in St. James' Park.”
Newcastle United head coach Eddie Howe added: “We are very happy that Malick is on board. He is a player I have admired for a long time and someone who will add real quality to our defensive options.
“Malick is still young, but brings valuable experience with Champions League football, and plays in the Bundesliga and Serie A, which is a great positive for us when we return to Europe this season.”