High extracellular matrix stiffness is a prominent feature of malignant tumors associated with poor clinical prognosis. To elucidate mechanistic connections between increased matrix stiffness and tumor progression, a variety of hydrogel scaffolds with dynamic changes in stiffness have been developed. These approaches, however, are not biocompatible at high temperature, strong irradiation, and acidic/basic pH, often lack reversibility (can only stiffen and not soften), and do not allow study on the same cell population longitudinally. In this work, we develop a dynamic 3D magnetic hydrogel whose matrix stiffness can be wirelessly and reversibly stiffened and softened multiple times with different rates of change using an external magnet. With this platform, we found that matrix stiffness increased tumor malignancy including denser cell organization, epithelial-to-mesenchymal transition and hypoxia. More interestingly, these malignant transformations could be halted or reversed with matrix softening (i.e., mechanical rescue), to potentiate drug efficacy attributing to reduced solid stress from matrix and downregulation of cell mechano-transductors including . We propose that our platform can be used to deepen understanding of the impact of matrix softening on cancer biology, an important but rarely studied phenomenon.