Bringing in a new centre-back was a priority for Newcastle this summer as manager Howe looks to increase competition and lower the average age of an experienced squad.
But getting deals over the line has been a tall order, with the Magpies missing out on several targets across all positions, including Benjamin Sesko, Hugo Ekitike, Joao Pedro and James Trafford.
Howe said: “We’re really pleased to have Malick on board. He’s a player I’ve admired for a long time and somebody who will add real quality to our defensive options.
“Malick is still young but brings valuable experience of Champions League football, as well as playing in the Bundesliga and Serie A, which is a major positive for us as we return to Europe this season.”
Thiaw, who has three caps for Germany, joined AC Milan from Schalke in 2022.
He made 31 appearances in all competitions for the Serie A giants last season, scoring his only goal of the campaign against Real Madrid in the Champions League in November.
Newcastle open their Premier League campaign at Aston Villa on Saturday.
