Due to its controlled reaction with water and biofluids, Mg as a dissolvable conductor has enabled the development of many transient electronic devices. In addition, Mg is a novel plasmonic material with high extinction efficiency, but its transientoptical properties have not been explored thoroughly. In this study, for the first time, we exploit the transient and tunable plasmonic properties of Mg in environmental and biomedical sensor applications. We used soft nanoimprint lithography to fabricate flexible and large-area Mg plasmonic structures that can be applied on the human skin. Their resonance (or color) can be tuned in the visible range by gradual Mg dissolution in a water fluid or vapor-rich environment; these structures can be easily implemented as passive optical sensors without the need for complex electronic circuits or a power supply. We demonstrate the applications of our optical sensors in the accurate monitoring of environmental humidity and physiological detection of sweat loss on the human skin during exercise. Our devices could be used as decomposable/resorbable optical sensors and can help minimize long-term health effects and environmental risks associated with consumer device waste, which will lead to many new possibilities in transient photonic device applications.