<p>Pursuing small catalyst sizes has remained a constant endeavor to maximize reaction yields by exposing more active sites and/or enhancing charge extraction. Organic particle photocatalysts typically remain tens to hundreds of nanometers in size, constrained by the quasi-infinite conjugation of commonly used polymers. Here, we reveal the size-minimized organic heterojunction nanoparticles by using all small molecule photovoltaic materials, and significantly enhance their photocatalytic activities for solar-driven hydrogen evolution. Owing to the intrinsically weak intermolecular forces and the absence of molecular entanglement of small molecules, (sub)nanometer-scale diameters of polymer-free nanoparticles are yielded, representing a size reduction of 1-2 orders of magnitude than traditional polymer-containing nanoparticles. Optimized polymer-free nanoparticles attaching on covalent frameworks achieve photocatalytic mass-united hydrogen evolution rates of up to 527.2 ~ 3180.7 mmol h⁻¹ g⁻¹ at varied concentrations under simulated sunlight, and the external quantum efficiency is up to 32.8 % at near-infrared light, marking a competitive performance for organic photocatalysts.</p>