A waste shredder (1) includes at least one loading chamber (3), two or more rotors (4) placed in seats formed in the bottom wall of the loading chamber (3) and a movable hopper (2) sliding above the loading chamber (3) and provided with a reciprocating translational movement. The hopper (2) has two end walls (B, B′) and carries a plurality of stiff vanes (5) placed in the spaces between the rotors (4), the end walls (B, B1) and the stiff vanes (5) protruding downward until they skim the bottom of the loading chamber (3) to press the material (8) to be shredded against the rotors (4). The rotors (4) turn in the opposite direction to the translational movement of the hopper (2) and reverse their direction of rotation when the hopper (2) reverses its direction of translation.