Bulk carbon materials can be synthesized through the modulation of precursor structures, stacking mode, and the optimization of pyrolysis conditions. The structure of two-dimensional (2D) carbon materials, however, is mainly driven by the pyrolysis temperature, pressure, and organic gas flow rates. Here, we report the synthesis of a centimeter-sized nanocrystalline graphene monolayer from pyrolyzing a Langmuir monolayer of a hexa(terpyridine)hexaphenylbenzene polyaromatic hydrocarbon (PAH) yielding a 2D carbon material with a thickness of 0.36 ± 0.07 nm. By designing the amphiphilic PAHs and reducing intermolecular π-π stacking interactions, a monolayer of horizontally aligned PAH was obtained at the water-air interface. During the pyrolysis, the monolayer served as a molecularly thin solid carbon source, leading to the controlled preparation of an electrically insulating nanocrystalline graphene monolayer. Our work proposes a 2D carbon material formed from the initial assembly of PAH monolayer at the water-air interface and subsequent pyrolysis and offers a modular strategy for designing nanocrystalline 2D films.