De Jong's younger brother Luuk spent six months on loan at St James' Park last season.
"I had been at Ajax a long time and wanted a new challenge in a new environment in a new country," added the former captain of the Amsterdam club.
De Jong, who can also operate as a striker, has won six caps for Netherlands but missed out on a place in the World Cup squad in Brazil.
He went through the youth system at Ajax before making his debut in 2007. He eventually appeared almost 250 times for the Dutch club, scoring 78 goals.
The transfer fee was not disclosed.
"It is a big signing for this club," said Newcastle manager Alan Pardew. "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."
Newcastle start the new season with a home game against league champions Manchester City on Aug. 16.
(Writing by Tony Jimenez, editing by Greg Stutchbury)
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