Harper, who will be responsible for features, will join the women's magazine in early February.
Harper's experience includes stints as a feature writer on the Daily Express and Mail on Sunday. She became features editor of Red in 2002 and was promoted to deputy editor of the middle-youth monthly glossy in 2003.
Grazia editor Jane Bruton said: "Harper is a tremendous signing for us as we take Grazia forward to the next stage of its development, giving our readers everything they have traditionally expected from a monthly but on a stylish and newsy weekly basis.
"She has a real feel for the issues which interest and concern the modern fashion-conscious women who are turning in increasing numbers to Grazia as their weekly bible."
Launched a year ago, Grazia posted an initial ABC six-monthly circulation figure of 650,000 copies in August.
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