Surface pressure is a fundamental thermodynamic property related to the activity of molecules at interfaces. In molecular simulations, it is typically calculated from its definition: the difference between the surface tension of the air−water and airsurfactant interfaces. In this Letter, we show how to connect the surface pressure with a two-dimensional osmotic pressure and how to take advantage of this analogy to obtain a practical method of calculating surface pressure−area isotherms in molecular simulation. As a proof-of-concept, compression curves of zwitterionic and ionic surfactant monolayers were obtained using the osmotic approach and the curves were compared with the ones from the traditional pressure tensor-based scheme. The results shown an excellent agreement between both alternatives. Advantageously, the osmotic approach is simple to use and allows to obtain the surface pressure−area isotherm on the fly with a single simulation using equilibration stages.