A modification of the Wilhelmy method is proposed for measuring the surface tension of polymer nanoparticle dispersions, whose key feature is the static nature of the Wilhelmy plate when measuring the surface tension force. Tests on the example of nanostructured chitosan aspartate showed satisfactory accuracy of the modified method in estimating the surface activity of nanodispersions in a wide range of polymer concentrations. The concentration dependence of the surface tension of the dispersion of chitosan aspartate nanoparticles was obtained, and concentration ranges with different surface effects were outlined. The contribution of individual components used in obtaining nanoparticles to the surface activity of the nanodispersion was estimated.