De Jong has travelled to the country and agreed personal terms. He has passed a medical after being given the go-ahead by Ajax.
The 25-year-old has penned a six-year deal at St James’ Park, just months after his brother Luuk De Jong had an unsuccessful loan in the North-East, and Newcastle have confirmed the transfer but not the fee.
The Ajax attacking midfielder has been on the Magpies’ radar for more than a year but a fresh move has proven successful because he has entered the final 12 months of his contract. Ajax, who have previously quoted clubs nearer to £15m, fear losing him for nothing next summer when he becomes a free agent.
The four-time Eredivisie champion has appeared almost 250 times for the Dutch giants, scoring 78 goals, and was appointed their captain in the summer of 2012.
De Jong said: “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 teammates and hopefully we can have a successful season.”
Newcastle United boss 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.”
While Newcastle boss Alan Pardew is keen to bring in a couple of new strikers this summer, he has also been determined to have a new creative spark in the middle following the departure of Yohan Cabaye in January.
De Jong should provide that, although he does not possess the sort of unpredictable flair Hatem Ben Arfa offers nor the composure and craft of a Cabaye. What he should provide, though, is goals.
He has captained Ajax, where he graduated through the club’s reputable academy system, and last season’s injury-hit campaign was the first time he had not hit double figures since 2009.
De Jong's arrival lessens the need for an agreement to be reached for Montpellier’s more expensive Remy Cabella, who is on duty with France at the World Cup.
Since stepping in to the professional ranks at Ajax he has played for Holland at every level and has six full caps for his country. He was not, though, included in Louis van Gaal’s squad for this summer’s finals in Brazil.
De Jong is also perfectly suited to playing as a striker, which could be where he starts the new Premier League season if Pardew was unable to bring in the likes of Alexandre Lacazette from Lyon. Newcastle have also, again, been credited with a move for Burnley's Danny Ings.