MoS2 has emerged as a promising alternative electrocatalyst for the hydrogen evolution reaction (HER) due to high intrinsic per‐site activity on its edge sites and S‐vacancies. However, a significant challenge is the limited density of such sites. Reducing the size and layer number of MoS2 and vertically aligning them would be an effective way to enrich and expose such sites for HER. Herein, a facile self‐limited on‐site conversion strategy for synthesizing monolayer MoS2 in a couple of nanometers which are highly dispersed and vertically aligned on 3D porous carbon sheets is reported. It is discovered that the preformation of well‐dispersed MoO3 nanodots in 1–2 nm as limited source is the key for the fabrication of such an ultrasmall MoS2 monolayer. As indicated by X‐ray photoelectron spectroscopy and electron spin resonance data, these ultrasmall MoS2 monolayers are rich in accessible S‐edge sites and vacancies and the smaller MoS2 monolayers the more such sites they have, leading to enhanced electrocatalytic activity with a low overpotential of 126 mV at 10 mA cm−2 and 140 mV at 100 mA mg−1 for HER. This state‐of‐the‐art performance for MoS2 electrocatalysts enables the present strategy as a new avenue for exploring well‐dispersed ultrasmall nanomaterials as efficient catalysts. An ultrasmall MoS2 monolayer well‐dispersed and vertically aligned on porous carbon is developed via a self‐limited on‐site fabrication strategy. It exhibits excellent and size‐dependent electrocatalytic performance for hydrogen evolution with an ultralow overpotential of 126 and 140 mV at 10 mV cm−2 and 100 mA mg−1, respectively.