The 23-year-old made 45 appearances in two seasons at Gladbach after joining from Twente in 2012, but had not started a single Bundesliga match this campaign and failed to register a league goal.
|9/1||Newcastle are 9/1 with bet365 to beat Sunderland 2-0|
"I'm really motivated," De Jong told the club's official website. "The first half of the season hasn't been nice for me because I didn't play much.
"I just want to show my talent, score goals and be successful again. That is why I'm here. My goalscoring record in Holland was very good and hopefully I can show that here."
De Jong may make his Newcastle debut in Saturday's Tyne-Wear derby against Sunderland and says he cannot wait to experience the atmosphere.
"I know how big it is and I love playing in these massive matches. That is why I became a footballer," he continued.
"It's always nice to be involved in a game like that and I would love to play a part. We have to win it because otherwise I know we cannot walk around the streets of Newcastle."