The 25-year-old, who can also operate as a striker, wrapped up his move to the Magpies after leaving Ajax’s pre-season training camp in Austria to undergo a medical and agree personal terms.
De Jong, who is seen as a replacement for Yohan Cabaye at St James’ Park, had one year remaining on his deal with the Dutch club but is keen to play in the Premier League.
He has spent his entire career with Ajax, for whom he was appointed captain in summer 2012, and has played in the Champions League and Europa League.
He has made six appearances for the Netherlands but is not part of the World Cup squad in Brazil.
His brother Luuk de Jong was on loan at Newcastle from Borussia Monchengladbach in the second half of last season but the Magpies decided against signing him on a permanent deal after he failed to impress.
After putting pen to paper, Siem de Jong, who will cost £6million according to some reports, told Newcastle's official website: "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."
Newcastle manager Alan Pardew said: "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.
"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."
You'll be able to watch action from the Eredivisie next season on Sky Sports 5.