The 18-year-old midfielder has signed a five-year deal with the Magpies after impressing while on trial at the club.
Reserve team manager Peter Beardsley said: "We are delighted to bring Mehdi to the club.
"He is a bright, young talent and we think he has an exciting future ahead of him."
Newcastle manager Alan Pardew has already signed Yohan Cabaye and Sylvain Marveaux and has also brought in striker Demba Ba from West Ham United.
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