Engineering the size, shape and crystallinity of organic semiconductor nanostructures (OSNs) offers the opportunity of fine tuning their optoelectronic properties for photocatalytic applications. Here, we report a facile and general ‘ice-melting’ kinetic control strategy to synthesize OSNs with excellent photocatalytic performance. The ultraslow release and interaction of organic molecules from ice melting yields an ultralow local concentration (∼five orders of magnitude fewer molecules), fundamentally enhancing the energy barrier thus leading to their relaxed and ordered assembly into refined nanocrystals (∼one order of magnitude smaller in size, morphological change from amorphous to crystalline); such nanocrystals display improved light-harvesting and charge transfer properties and consequent photocatalytic efficiency towards degrading organic pollutants compared to standard OSNs, or even commercial titania photocatalysts (Degussa P25); we also generalize the ‘ice-melting’ kinetic control strategy to synthesize a series of highly photoactive organic nanocrystals.