An automatically retractable pet leash, comprises a casing or guard (10) inside which is a winding device of a cord or ribbon that protrudes from the casing or guard (10) through an aperture (13), and a braking mechanism operated by a pushbutton (12), and also comprises a handle (11) for gripping the leash with one hand. It also comprises a protective shell (14) designed to protect the casing or guard (10) from knocks or blows that can damage it； the casing or guard comprises at least one housing designed to accommodate by snap-fitting, when used, at least one coupling or ribbing element foreseen on the body of the protective shell (14) in such a way as to hold the protective shell (14) locked against the casing or guard (10).
