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.
"We are absolutely delighted to have finally secured the signature of Mathieu," Newcastle manager Alan Pardew told the club's website (www.nufc.co.uk).
"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."
Debuchy's signing is a boost for the club as they try and arrest a worrying slide towards the relegation zone after six defeats in their last seven league games.
Three consecutive defeats over the holiday period, in which they conceded 13 goals, has left Newcastle two points above third from bottom Wigan Athletic.
Newcastle, who have been struggling with a long injury list and uncertainty about the future of striker Ibrahim Ba, are away to Championship (second division) side Brighton and Hove Albion in the third round of the FA Cup on Saturday. (Reporting by Martyn Herman, editing by Alan Baldwin)