The deal to bring the Italy midfielder is worth a reported £52million, with the 23-year-old signing until 2028 at St James' Park.
The young midfielder arrives in the Premier League having played 130 times for Milan and with 14 senior Italy caps, with his performances seeing him compared to midfield maestro Andrea Pirlo.
Tonali initially joined Milan from boyhood club Brescia on loan in 2020, before making his move permanent a year later.
He helped the Rossoneri win the Scudetto in the 2021/22 campaign, and made 48 appearances for the club in all competitions last season as they finished fourth in Serie A and reached the Champions League semi-finals.
"First of all, I want to thank Newcastle United because they are giving me a huge opportunity for my career," he said.
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"I want to repay the trust on the pitch, giving it my all, as I always have.
"I'm really excited about playing at St James' Park. I can't wait to feel the warmth of the fans."
Magpies boss Eddie Howe added: "I'm delighted to welcome Sandro to Newcastle United.
"He is an exceptional talent and has the mentality, physicality and technical attributes to be a great fit for us.
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"At 23, Sandro already has important experience as a key player in one of Europe's top leagues and in the Champions League, as well as playing for his country.
"But he also has the opportunity and potential to grow and evolve with us, and I'm excited to add him to our squad as we approach the exciting season ahead."
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