Since the advent of double network (DN) hydrogels nearly 20 years ago, they have flourished as smart soft materials. Their two unique contrasting interpenetrating network structures and adjustable network crosslinking methods overcome the obstacles of single network hydrogels in mechanical properties. Thus, DN hydrogels with remarkable mechanical properties, excellent hydrophilicity, fast self-healing, high conductivity, and rapid stimulus responsiveness have been considered candidates for energy and environmental applications. In this review, we classify these types of hydrogels according to their components and discuss the most contemporary trends in their preparation methods, network structures, mechanical features, and self-healing mechanisms. Besides, we provide the future perspectives of DN hydrogels for energy applications, covering supercapacitors and batteries, and environmental applications including adsorption, solar interface evaporation, and catalytic degradation, and discuss some potential future directions.