The 23-year-old Dutch international, who was prolific for FC Twente before moving to Germany in 2012, will sport the number 18 shirt and could make his debut against Sunderland in the Tyne-Wear derby on Saturday.
"Newcastle is a great club and I have a good feeling about it," said De Jong. "I am really looking forward to playing here.
"I came here six years ago with my uncle and we were really impressed by the atmosphere, the fans and everything.
"I've now been to the training ground, seen the stadium inside and everything looks pretty good."
And the forward confirmed he will definitely be available for the Sunderland game.
"I feel good," he said. "I have no problems. We had a training camp in Germany with Borussia Monchengladbach. I felt good, I played 90 minute games, and I am ready to play."
Newcastle's director of football Joe Kinnear added: "We are delighted to bring Luuk de Jong to the club.
"He has an excellent pedigree and will add significantly to our striking options.
"We have been watching him for a little while now and I am sure he will prove to be a valuable acquisition for us.
"He's a great character and a proven goalscorer, who I know is very keen to replicate the prolific form he showed for FC Twente in the Eredivisie."