Abstract: The nonproportional multiaxial ratchetting of cast AZ91 magnesium (Mg) alloy was examined by performing a sequence of axial–torsional cyclic tests controlled by stress with various loading paths at room temperature (RT). The evolutionary characteristics and path dependence of multiaxial ratchetting were discussed. Results illustrate that the cast AZ91 Mg alloy exhibits considerable nonproportional additional softening during cyclic loading with multiple nonproportional multiaxial loading paths; multiaxial ratchetting presents strong path dependence, and axial ratchetting strains are larger under nonproportional loading paths than under uniaxial and proportional 45° linear loading paths; multiaxial ratchetting becomes increasingly pronounced as the applied stress amplitude and axial mean stress increase. Moreover, stress–strain curves show a convex and symmetrical shape in axial/torsional directions. Multiaxial ratchetting exhibits quasi-shakedown after certain loading cycles. The abundant experimental data obtained in this work can be used to develop a cyclic plasticity model of cast Mg alloys.