Here, we report a simple and general approach to fabricate free-standing two-dimensional (2D) sheets of nanoparticles by the simultaneous self-assembly of hydrophobic nanoparticles and hydrophilic polymers at the liquid-liquid interface. The nanoparticle-polymer interaction at the interface generates well-defined 2D sheets of densely packed nanoparticles with a lateral dimension of tens of micrometers. The nanosheets transferred in water are stable over months without any additional cross-linking step. The method is applicable for a broad range of nanoparticles including oxide, semiconductor, and metal nanoparticles as well as functional polymers.