Two-dimensional polymers (2DPs) are covalent polymeric networks with intrinsic long-range order in two orthogonal directions. Their unique physicochemical properties include tunable opto-electronic properties, mechanical stability and precisely defined nanopores that enable ion and molecular sieving. Here we describe a universal approach for the precise synthesis of 2DPs through the surfactant-monolayer-assisted interfacial synthesis method. The surfactant-monolayer-assisted interfacial synthesis combines the on-water surface reactivity and the role of surfactant (for example, sodium oleyl sulfate and sodium dodecyl benzenesulfonate) to guide the monomer-programmed assembly, two-dimensional (2D) polymerization and crystallization processes on the water surface, yielding large-area and highly crystalline 2DPs. This versatile Protocol has been used to synthesize many 2DP crystals, including 2D polyamide, 2D polyimide, 2D polyimine, 2D poly(boronate ester), 2D poly(pyridinium salt), 2D polyphenylenevinylene, 2D polybenzimidazole and 2D polyaniline, with a large lateral size of ~28 cm². The crystals have precisely tunable thicknesses ranging from ~1 to 200 nm by adjusting the reaction time and/or reactant concentration and large crystal domain (that is, the lateral size of a single crystalline region) up to ~160 μm², which helps minimize grain boundaries and preserve the intrinsic properties of 2DPs. Synthesis typically takes 2–14 days, depending on the reaction type. Film collection via a horizontal dipping method and a simple washing procedure, yield the target 2DPs. The resulting 2DP crystals have been integrated into electronics, optoelectronics, membranes and energy conversion/storage devices, exhibiting excellent performance. This Protocol can be performed easily by researchers with intermediate expertise in framework materials and interfacial chemistry.