This study investigated the transformation-induced plasticity (TRIP) effect in Mg–Sc alloys with β single-phase (body-centered cubic) structure. A Mg-19.8 at% Sc alloy showed fracture strain of ∼53 % accompanied by necking propagation with increasing the tensile strain at room temperature. An X-ray diffraction analysis and transmission electron microscopy observations confirmed the existence of a hexagonal close-packed phase beside the β matrix phase during and after the tensile test; this indicates a strain-induced martensitic transformation, i.e., the TRIP effect induced superior ductility. Moreover, the study revealed a threshold Sc content of ∼19.5 at% Sc at which the deformation behavior at room temperature changes from superelasticity to TRIP. The TRIP Mg–Sc alloys exhibited fracture strain from 45 % to 66 % and ultimate tensile strength (UTS) above 220 MPa to overcome the tradeoff between the fracture strain and UTS of conventional Mg alloys.