The 24-year-old Holland international midfielder completed his move from PSV Eindhoven on Saturday morning for a fee understood to be in the region of 20 million euros, around £14.4million.
Wijnaldum said: "I'm delighted to be a Newcastle United player.I felt the time was right for me to move and the Club have shown they really want me, so I'm very happy to be here."
Only Michael Owen and Alan Shearer have cost the Magpies more.
Wijnaldum, who becomes the fifth Dutch player on the books at St James' Park along with Tim Krul, Daryl Janmaat, Vurnon Anita and Siem de Jong, said: "There are a few players here who are from Holland - I know them well and they will help me feel at home in Newcastle.
"I know the club has a great stadium with great fans. I want to play here as soon as possible - I want to feel the atmosphere and hear the crowd. I cannot wait to get started."