Despite rapid progress in recent years, it has remained challenging to prepare crystalline two-dimensional polymers. Here, we report the controlled synthesis of few-layer two-dimensional polyimide crystals on the surface of water through reaction between amine and anhydride monomers, assisted by surfactant monolayers. We obtained polymers with high crystallinity, thickness of ~2 nm and an average crystal domain size of ~3.5 μm2. The molecular structure of the materials, their grain boundaries and their edge structures were characterized using X-ray scattering and transmission electron microscopy techniques. These characterizations were supported by computations. The formation of crystalline polymers is attributed to the pre-organization of monomers at the water-surfactant interface. The surfactant, depending on its polar head, promoted the arrangement of the monomers-and in turn their polymerization-either horizontally or vertically with respect to the water surface. The latter was observed with a surfactant bearing a carboxylic acid group, which anchored amine monomers vertically through a condensation reaction. In both instances, micrometre-sized, few-layer two-dimensional polyamide crystals were grown.