Prussian blue (PB) represents a simple, economical, and eco-friendly system as cathode material for sodium-ion batteries (SIBs). However, structural problems usually worsen its experimental performance thus motivating the search for alternative synthetic strategies and the formation of composites that compensate these deficiencies. Herein, a straightforward approach for the preparation of PB/MoS2-based nanocomposites is presented. MoS2 provides a 2D active support for the homogeneous nucleation of porous PB nanocrystals, which feature superior surface areas than those obtained by other methodologies, giving rise to a compact PB shell covering the full flake. The nanocomposite exhibits an excellent performance as cathode for SIBs with discharge capacity values up to 177 mA h g−1 and a specific capacitance of 354 F g−1. These values are even larger for the intercalation of K+ ions (up to 215 mA h g−1, reaching a specific capacitance of 489 F g−1). Compared to similar composites, superior performance can be ascribed to a synergistic effect of the coordination compound with the 2D material.