Newcastle said they have an option to make the deal permanent at the end of the season and that the 23-year-old is eligible to make his debut in Saturday's Tyne-Wear derby against Sunderland.
"Newcastle is a great club and I have a good feeling about it," De Jong, who has seven caps for the Netherlands, said on the club website.
"I am very impressed with the stadium and training ground and have been made to feel very welcome by everyone I have met."
The Dutchman joined Gladbach from Dutch side Twente in 2012 for a reported club record £10m but failed to deliver.
This season he dropped down the pecking order with the arrival of Raffael and Germany international Max Kruse and has not started any league game.
"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," he added.