Deep Earth hydrogen plays an important role in the structural evolution of our planet. This session focuses on the origin, speciation, and abundance of hydrogen in the Earth's deep interior, and its profound physical and chemical impacts on host minerals. We invite contributions exploring how deep hydrogen influences mineral phase transitions, rheology, electrical conductivity, and elastic wave velocities. Particular attention will be given to the role of hydrogen in shaping key deep Earth structures and interfaces, including the mantle transition zone, core–mantle boundary, and related discontinuities. We welcome interdisciplinary studies integrating high-pressure and high-temperature experiments, first-principles calculations, geophysical observations, and geochemical tracers. By bringing together diverse perspectives, this session aims to advance our understanding of deep hydrogen cycling, its incorporation mechanisms, and broader geodynamic and geophysical consequences.