Anita, a key member of the Ajax team that won back-to-back league titles in 2011 and 2012, has signed a five-year contract.
"I won almost everything in Holland so the step was not difficult to make. Now I think we can also win trophies with Newcastle and it's time to do that," Anita told the Newcastle website.
Newcastle have not won a major trophy since lifting the 1969 Fairs Cup, the precursor to the UEFA Cup which became the Europa League.
Manager Alan Pardew said Anita, a versatile player who is set to feature in a defensive midfield role, was raring to go.
"He's up to speed and ready to play when I need him. At 23 years old, we hope his best years are at Newcastle," said Pardew, who guided the north east England club to fifth in the top flight last season.
Anita, who made his debut for Ajax in 2006 aged 16 and also won the 2010 Dutch Cup, said fellow Dutch international and Newcastle goalkeeper Tim Krul had convinced him to join.
"He told me to join and said it was a very nice club with great supporters," said Anita.
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