The structure–property engineering of phase‐based materials for redox‐reactive energy conversion and environmental decontamination nanosystems, which are crucial for achieving feasible and sustainable energy and environment treatment technology, is discussed. An exhaustive overview of redox reaction processes, including electrocatalysis, photocatalysis, and photoelectrocatalysis, is given. Through examples of applications of these redox reactions, how structural phase engineering (SPE) strategies can influence the catalytic activity, selectivity, and stability is constructively reviewed and discussed. As observed, to date, much progress has been made in SPE to improve catalytic redox reactions. However, a number of highly intriguing, unresolved issues remain to be discussed, including solar photon‐to‐exciton conversion efficiency, exciton dissociation into active reductive/oxidative electrons/holes, dual‐ and multiphase junctions, selective adsorption/desorption, performance stability, sustainability, etc. To conclude, key challenges and prospects with SPE‐assisted redox reaction systems are highlighted, where further development for the advanced engineering of phase‐based materials will accelerate the sustainable (active, reliable, and scalable) production of valuable chemicals and energy, as well as facilitate environmental treatment.