The former Twente striker failed to make an impact in the Bundesliga since joining Gladbach in 2012, but is "looking forward" to getting his career back on track
The 23-year-old made the switch to the Bundesliga club in July 2012, after scoring 25 goals in 31 Eredivisie appearances for Twente in the previous campaign.
Newcastle are now giving the forward the chance to resurrect his career in the Premier League, and he is relishing the prospect of playing for the Magpies.
“Newcastle is a great club and I have a good feeling about it,” De Jong said in a statement.
“I am very impressed with the stadium and training ground and have been made to feel very welcome by everyone I have met.
“It has been a tough season for me so far but I am here to play games and want to be important for the team, and successful with goals and assists.
“I am really looking forward to playing for Newcastle.”
He will wear the No.18 shirt and is eligible to make his debut in the Tyne-Wear derby against rivals Sunderland on Saturday.
The Magpies announced the deal for De Jong shortly after Yohan Cabaye was officially announced as a Paris Saint-Germain player.
Pardew could further bolster his ranks before the close of the transfer window on Friday, with Newcastle rumoured to be interested in a deal for Lyon midfielder Clement Grenier and Montpelllier's Remy Cabella.