There is a growing interest in engineering proteins whose function can be controlled with the spatial and temporal precision of light. Here, we present a novel example of a functional light-triggered switch in the Ca-dependent cell-cell adhesion protein E-cadherin, created using a mechanism-based design strategy. We report an 18-fold change in apparent Ca2+ binding affinity upon illumination. Our results include a detailed examination of functional switching via linked changes in Ca2+ binding and cadherin dimerization. This design opens avenues toward controllable tools that could be applied to many long-standing questions about cadherin's biological function in cell-cell adhesion and downstream signaling.