Layered 2D materials such as transition metal chalcogenides are promising electrocatalysts for hydrogen evolution reaction (HER) due to the flexible compositions and distinctive electronic structures. However, their active sites usually stem from the edges, whereas the basal planes with the higher surface area are difficult to activate for water dissociation and H₂ evolution. Here, a novel quasi-2D AgRuO₃ compound, which can be readily activated by cyclic voltammetry and split into layered structures with more exposed basal planes is reported. This results in an outstanding HER activity with a low overpotential of only 37 mV at 10 mA cm⁻² and a Tafel slope of 36 mV dec⁻¹. It is found that the oxygen vacancies generated on the basal planes during activation can thermodynamically facilitate water adsorption, dissociation, and intermediate OH* desorption compared with the pristine AgRuO₃, as revealed by theoretical calculations. Thus, the oxygen vacancies on the exposed basal planes are the active centers. This work sheds light on the evolution of a quasi-2D metal oxide during HER and highlights the active role of basal planes that can facilitate water dissociation in alkaline water electrolysis.