Huntelaar was the subject of constant media speculation after struggling to nail down a starting place in his first season at the San Siro last term following his move from Real Madrid.
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The transfer of Sweden striker Zlatan Ibrahimovic from Barcelona to Milan paved the way for Huntelaar's switch to Schalke.
"It is true I never wanted to leave Milan but that changed when Zlatan Ibrahimovic came from Barcelona," the Dutchman told his website.
"Schalke and Milan have agreed a transfer fee of about 13 million euros."
Schalke, who are playing in the Champions League this season, have lost both their opening Bundesliga matches.
TOUGH NEGOTIATIONS
The 27-year-old Huntelaar played for four years at Ajax Amsterdam before moving in 2009 to Real, where he also failed to establish himself.
"Negotiations were tougher than expected. I am happy to have Klaas-Jan. He will help us," said Schalke coach Felix Magath.
Jurado, 24, started his career with Real's youth team, playing briefly with the seniors before joining Atletico in 2006.
"Atletico has reached an agreement for the transfer of Jurado," the club said on their website.
Plestan, 29, will become the first Frenchman to play at Schalke and a much-needed addition to a central defence that has been far from convincing this season, despite the signing of former Real player Christoph Metzelder.
The club said Plestan had signed a three-year contract.
The new recruits join former Real striker Raul, who moved to last season's Bundesliga runners-up earlier in the transfer window.
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