The 27-year-old has signed a five-and-a-half-year contract and joins the Magpies for an undisclosed fee.
Debuchy told the club's official website: "It is great to become a Newcastle United player.
"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.
"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 give everything for this Club."
Newcastle manager Alan Pardew said: "We are absolutely delighted to have finally secured the signature of Mathieu.
"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 the team.
"At the same time it's a huge relief to actually have him with us now.
"He is an established top international player and it is a real coup for this Club that we now have him at St. James' Park.
"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 too."