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Hydrogen has long been regarded as a detrimental interstitial element in metallic alloys, primarily owing to its role in hydrogen embrittlement. However, recent experimental and computational studies, principally involving austenitic steels including Fe–Cr–Ni stainless steels, high-Mn TWIP alloys, and low-density steels (alloyed with aluminum) have revealed that hydrogen can, under carefully controlled conditions of alloy composition, microstructure, stacking fault energy, and hydrogen concentration, act as a functional and beneficial interstitial solute element that can enhance strength, ductility, or even both. This review critically examines the fundamental mechanisms underlying this paradoxical yet promising behavior, with particular emphasis on hydrogen-enhanced twinning, hydrogen-enhanced densified twinning, hydrogen-enhanced microbanding, and hydrogen solid-solution strengthening. The roles of stacking fault energy, grain size, precipitation (κ-carbide, B2 intermetallic), and hydrogen charging protocol (electrochemical vs. gaseous charging; pre-charging vs. in-situ) on the operative mechanism and net mechanical response are systematically discussed. A key conclusion is that the transition from hydrogen embrittlement to hydrogen-enhanced mechanical properties is governed by a multi-dimensional parameter space that has not yet been fully mapped. These findings provide a scientific basis for a conceptual reorientation in designing alloy compositions and microstructures that may promote beneficial hydrogen–defect interactions under controlled or even in-service hydrogen exposure while suppressing embrittlement.