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 said in a statement: "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."
The move came hours after Newcastle tabled a bid described by club insiders as "substantial" for Lyon midfielder Clement Grenier, a man they have scouted as a possible replacement following the sale of fellow France international Yohan Cabaye to Paris St Germain.
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.
Yoan Gouffran, who has been playing in a wide midfield position for much of the season but is a striker by trade, was also conspicuous by his absence at Carrow Road because of a fitness concern.
Shoulder 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.
However, De Jong said: "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."
The Dutchman has been in Newcastle's sights for some time, with the Tyneside club having first tried to land him 18 months ago before he opted for the Bundesliga.
They now hope a career which has stalled somewhat can be revived in England.
Director of football Joe Kinnear said: "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."
De Jong's arrival was already planned before Cabaye's big-money departure for PSG, and Pardew has left owner Mike Ashley and Kinnear in little doubt that he expects him to be replaced ahead of Friday night's 11pm deadline.
That, of course, did not happen when striker Andy Carroll left the club in a £35million move to Liverpool on the final day of the window in January 2011.
But this time around, the Magpies have slightly more time to implement the contingency plan they drew up just in case they did receive an offer they could not refuse for the £20million-rated Cabaye, and they have moved swiftly to do so.
Grenier, who has scored 12 goals in 89 appearances for Lyon, was one of a series of potential targets identified and Newcastle are now working to thrash out a deal which would see the 23-year-old plug the sizable gap left in Pardew's team.
However, his current manager Remi Garde has voiced his hope that the player will not be sold this month, and it remains to be seen whether or not the Magpies can tempt the Ligue 1 club to part with him.
Meanwhile, Newcastle have insisted Cabaye left with their best wishes as he sought a return to Champions league football.
A spokesman said: "Newcastle United have benefited significantly from Yohan's talents since his arrival from Lille in the summer of 2011, and the player has likewise benefited, developing hugely during his time on Tyneside.
"The club wanted to keep him but the lure of Champions League football and the prospect of being involved in the sporting project at PSG was an opportunity that Yohan made very clear to us he wanted to take.
"Yohan leaves the club with our best wishes for his future at PSG and, hopefully, as part of the French national squad for this summer's World Cup."