Coordination polymers (CPs) have attracted significant attention due to their structural diversity and tunable properties, enabling applications in areas such as gas storage, adsorption, and sensing. While most CPs employ transition metal centers, alkaline earth metals offer advantages including low toxicity, low molecular weight, and cost effectiveness. Here, we report magnesium(II)-based coordination polymers focusing on their structural characteristics and solid-state [2+2] photocycloaddition behavior. Upon UV irradiation, the Mg(II) CPs undergo photocycloaddition reactions in a single-crystal-to-single-crystal (SCSC) manner, allowing direct observation of structural changes. Subsequent thermal treatment induces a reversible transformation that restores the initial crystal structure. These results highlight the potential of Mg(II)-based coordination polymers as reversible photoresponsive solid-state materials.