The development of cost-effective and efficient catalysts via an environmentally friendly synthetic process has significance for future renewable energy storage and conversion systems. Molybdenum disulfide (MoS2) is a promising 2D-material and has received increasing attention due to its high HER activity. Still, the HER activity of MoS2 remains challenging in a wide pH range. Here, we demonstrate that small size MoS2 quantum dots (MSQDs) can serve as a highly electrochemically active and low-cost electrocatalyst for HER over a wide pH range (pH 0-14). The bottom-up strategy was developed to synthesize water-soluble MSQDs from a carbon-free single molecular precursor, avoiding the formations of unwanted carbon quantum dots (CQDs). By taking the structural benefit, the as-synthesized MSQDs exhibit lower overpotentials and a smaller Tafel slope in acidic, basic, and neutral medium. The catalysts show long-term stability in the entire pH range. This study provides a noble strategy to synthesize MSQDs for optimizing its HER activity.