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The quantitative correlation of the catalytic activity with the microscopic structure of heterogeneous catalysts is a major challenge for the field of catalysis science. It requests synergistic capabilities to tailor the structure with atomic scale precision and to control the catalytic reaction to proceed through well-defined pathways. Here we leverage on the controlled growth of MoS₂ atomically thin films to demonstrate that the catalytic activity of MoS₂ for the hydrogen evolution reaction decreases by a factor of ∼4.47 for the addition of every one more layer. Similar layer dependence is also found in edge-riched MoS₂ pyramid platelets. This layer-dependent electrocatalysis can be correlated to the hopping of electrons in the vertical direction of MoS₂ layers over an interlayer potential barrier. Our experimental results suggest the potential barrier to be 0.119 V, consistent with theoretical calculations. Different from the …