The Premier League club have the option to make the deal permanent when it expires in the summer.
The Dutchman, 23, is available for Saturday's derby against Sunderland at St James' Park.
"Newcastle is a great club and I have a good feeling about it," De Jong told the club's official website.
"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."
De Jong joins Newcastle having failed to score in 13 games for Monchengladbach this season.