The 23-year-old, who will wear the number 18 shirt, underwent a medical on Tyneside on Wednesday before formally completing a temporary switch to the Magpies, who have also negotiated an option to buy the player in the summer.
De Jong: "Newcastle is a great club and I have a good feeling about it. 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."
De Jong's arrival could hardly have proved more timely with leading scorer Loic Remy set to sit out the club's next three games after being sent off during Tuesday night's frustrating 0-0 draw at Norwich, a fixture fellow frontman Papiss Cisse missed with a back injury.
Should neither he nor Cisse recover in time for Saturday's derby clash with Sunderland at St James' Park, manager Alan Pardew would face a major decision over whether or not to involve De Jong, who has not made a single start for Gladbach so far this season having been cast in the role of perennial late - and more often than not, very late - substitute.