Semiconductor photocatalysts absorb light and convert it to photogenerated charges that can drive redox reactions on their surface. Organic semiconductors are increasingly being employed for photocatalytic applications, however, photocatalysts formed from a single organic semiconductor typically suffer from inefficient intrinsic charge generation, which leads to low photocatalytic activities. We demonstrate that fabricating organic nanoparticle (NP) photocatalysts that contain a heterojunction between a donor polymer (PTB7-Th) and non-fullerene acceptor (EH-IDTBR) can result in H2 evolution photocatalysts with greatly enhanced photocatalytic activity. Control of the nanomorphology of these NPs was critical to achieving optimum H2 evolution, and was achieved by varying the stabilizing surfactant employed during NP fabrication. Converting the morphology from a core-shell structure to an intermixed donor/acceptor blend increased photocatalytic activity by an order of magnitude, resulting in H2 evolution photocatalysts that are among the most active reported to date, with a H2 evolution rate of over 60,000 µmolh-1g-1 under 350 to 800 nm illumination and external quantum efficiencies over 6% in the region of maximum solar photon flux. Kosco, J. et al. Nat. Mater. In Press. DOI: 10.1038/s41563-019-0591-1