Black phosphorus quantum dots (BP QDs) are facilely loaded on MXene nanosheets through van der Waals self-assembly. On the one hand, the BP QDs possess large surface areas and abundant active sites, offering fairly high electrocatalytic activity. On the other hand, the MXene nanosheets serve as a 2D substrate to confine the BP QDs, preventing them from aggregation and, thus, electrolyte inaccessibility. They can further provide excellent electronic conductivity and superior structural robustness. The BP QDs, in turn, act as a spacer to isolate the nanosheets from restacking, thus preserving the active sites from being lost. The resulting BP QDs/MXene nanohybrids, as a bifunctional electrocatalyst, exhibit remarkable synergy in both hydrogen and oxygen evolution reactions (HER/OER) in alkaline media. Specifically, an overpotential of 360 mV ( vs. RHE) and a Tafel slope of 64.3 mV dec −1 are achieved for the OER, and an overpotential of 190 mV ( vs. RHE) and a Tafel slope of 83.0 mV dec −1 for the HER in 1.0 M KOH solution are achieved. These values are significantly lower than those of their components (BP QDs and MXene nanosheets) and even approach those of commercialized RuO 2 or Pt/C. When the BP QDs/MXene nanohybrids are employed as both the cathode and anode in a full cell for overall water splitting, a current density of 10 mA cm −2 is quickly reached at a potential of only 1.78 V. Additionally, the underlying mechanism of the superior electrocatalytic performance of the BP QDs/MXene nanohybrids is fundamentally explored by density functional theory calculations.