Existing optical imaging techniques offer us powerful tools to directly visualize the cellular structure at the microscale; however, their capability of nanoscale sensitivity is restricted by the diffraction-limited resolution. We show that the mesoscopic light transport theory analysis of the spectra of partial waves propagating within a weakly disordered medium, such as biological cells [i.e., partial wave spectroscopy (PWS)] quantifies refractive index fluctuations at subdiffractional length scales. We validate this nanoscale sensitivity of PWS using experiments with nanostructured models. We also demonstrate the potential of this technique to detect nanoscale alterations in cells from patients with pancreatic cancer who are otherwise classified as normal by conventional microscopic histopathology.