The experienced right-back, signed for an undisclosed fee, has been a mainstay of Lille's defence since emerging from the youth ranks 10 years ago, making more than 230 appearances.
However, after signing a contract until summer 2018, the 27-year-old said he was looking forward to helping Newcastle climb the Premier League table after a worrying loss of form.
"After many great years at Lille I felt I was ready for a new challenge, and cannot wait to play for such a big club in the best league in the world," he told the club's website.
"I hope I can bring something fresh to the team and help us get up to the highest possible position in the Premier League this season.
"I will play like I used to for Lille, with the same qualities. I don't leave anything on the field and give more, and I hope I will bring a lot to this team."
Debuchy's signing is a boost for manager Alan Pardew whose injury-hit side have lost six of their last seven games and who have fallen to within two points of the relegation zone.
They conceded 13 goals over the holiday programme, including a 7-3 defeat at Arsenal, and have had to deal with uncertainty about the future of striker Demba Ba who completed his move to Chelsea on Friday.
"We are absolutely delighted to have finally secured the signature of Mathieu," Pardew said.
"It's no secret we've been trailing him for some time now and to have reached this point fills me with great excitement and anticipation in respect of what he will be able to offer.
"He is an established top international player and it is a real coup for this club. Mathieu will add real quality to our team, in terms of his ability, character and experience and I am sure he will quickly become a firm favourite among our fans."
Newcastle are away to Championship side Brighton and Hove Albion in the third round of the FA Cup on Saturday.
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