Carbon nanostructures that feature two-dimensional extended nanosheets are important components for technological applications such as high-performance composites, lithium-ion storage, photovoltaics and nanoelectronics. Chemical functionalization would render such structures better processable and more suited for tailored applications, but typically this is precluded by the high temperatures needed to prepare the nanosheets. Here, we report direct access to functional carbon nanosheets of uniform thickness at room temperature. We used amphiphiles that contain hexayne segments as metastable carbon precursors and self-assembled these into ordered monolayers at the air/water interface. Subsequent carbonization by ultraviolet irradiation in ambient conditions resulted in the quantitative carbonization of the hexayne sublayer. Carbon nanosheets prepared in this way retained their surface functionalization and featured an sp(2)-rich amorphous carbon structure comparable to that usually obtained on annealing above 800 degrees C. Moreover, they exhibited a molecularly defined thickness of 1.9 nm, were mechanically self-supporting over several micrometres and had macroscopic lateral dimensions on the order of centimetres.