A corner deflector of a fitting for a window casement or a door leaf, insertable into a C-shaped casement/leaf groove in the rebate of a frame profile of the casement/leaf. The corner deflector having a first corner limb and a second corner limb at right angles to the first corner limb. A groove for a limb of the casement/leaf groove, the limb groove having an undercut, is provided on a first longitudinal side of the first corner limb and/or of the second corner limb. The region of the second longitudinal side of the first and/or second corner limb which is located opposite the undercut of the first longitudinal side is undercut-free. At least one securing element is provided on the second longitudinal side, which can be moved relative to the second longitudinal side between a securing position and a disengagement position.