The 25-year-old Holland international moves to Tyneside for an undisclosed fee and has signed a six-year deal at St James' Park.
De Jong joins Tenerife striker Ayoze Perez and Sunderland midfielder Jack Colback in joining the Magpies this summer as boss Alan Pardew looks to win over the fans following what was seen by many as a disappointing campaign last term.
"We are absolutely overjoyed to bring Siem to the club because he will give us intelligence in the final third and create goalscoring situations, which we lacked towards the end of last season," Pardew said.
"It is a big signing for this club. Siem brings massive experience from Ajax. I think our fans are going to enjoy the way he plays and I cannot wait to start working with him."
De Jong remarked: "It is great to have signed for Newcastle.
"It is a big club which my brother told me all about, so I already feel good here.
"I had been at Ajax a long time and wanted a new challenge in a new environment and a new country.
"Coming to Newcastle is a great opportunity and the chance to play for a club like this in the Premier League was one I could not turn down.
"I am looking forward to starting training and meeting my new team-mates and hopefully we can have a successful season."
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