Transition metal sulfides (TMSs) have emerged as one of the most promising classes of electrocatalysts for the oxygen evolution reaction (OER) because of their excellent electrical conductivity, structural versatility, and earth-abundant composition. Their tunable crystal structures ranging from layered MS2 to non-layered MxSγ systems enable flexible control of electronic states and active-site density. This chapter comprehensively examines the synthesis, structure–property relationships, and catalytic mechanisms of both amorphous and crystalline TMSs, emphasizing their roles as active or precursor species in alkaline OER. Particular attention is devoted to their in situ transformation into (oxy)hydroxides, which generate new high-valence metal sites and enhance catalytic activity. The chapter also outlines advances in top-down and bottom-up fabrication routes, including exfoliation, hydrothermal growth, and vapor-phase deposition, each offering pathways to control morphology, crystallinity, and defect concentration. The impact of sulfur chemistry, heteroatom doping, interface engineering, and hybridization with conductive supports is discussed as key strategies for improving kinetics and durability. Furthermore, the comparative study between amorphous and crystalline TMSs elucidates the significance of short-range disorder and lattice periodicity in determining catalytic behavior. Collectively, these insights provide design principles for next-generation, cost-effective TMS-based OER electrocatalysts with high activity, stability, and scalability.