[EN] Full measurement of the polarization of light at the nanoscale is expected to be crucial in many scientific and technological disciplines. Ideally, such measurements will require miniaturized Stokes polarimeters able to determine polarization nondestructively, locally, and in real time. For maximum robustness in measurement, the polarimeters should also operate optimally. Recent approaches making use of plasmonic nanostructures or metasurfaces are not able to fulfill all these requirements simultaneously. Here, we propose and demonstrate a method for subwavelength-footprint Stokes nanopolarimetry based on spin-orbit interaction of light. The method, which basically consists on a subwavelength scatterer coupled to a (set of) multimode waveguide(s), can fully determine the state of polarization satisfying all the previous features. Remarkably, the nanopolarimetry technique can operate optimally (we design a nanopolarimeter whose polarization basis spans 99.7% of the maximum tetrahedron volume inside the Poincaré sphere) over a broad bandwidth. Although here experimentally demonstrated on a silicon chip at telecom wavelengths, spin-orbit interaction-based nanopolarimetry is a universal concept to be applied in any wavelength regime or technological platform. A.M. acknowledges support from the Spanish Ministry of Economy and Competiveness (MINECO) under grant TEC2014-51902-C2-1-R and the Valencian Conselleria d'Educacion, Cultura i Esport under grant PROMETEOII/2014/034. FJ.R.-F. acknowledges support from the European Research Council under project ERC-2016-STG-714151-PSINFONI. A.E.-S. acknowledges support from the Spanish Ministry of Economy and Competiveness (MINECO) under grant BES-2015-073146. Espinosa Soria, A.; Rodríguez Fortuño, FJ.; Griol Barres, A.; Martínez Abietar, AJ. (2017). On-Chip Optimal Stokes Nanopolarimetry Based on Spin-Orbit Interaction of Light. Nano Letters. 17(5):3139-3144. https://doi.org/10.1021/acs.nanolett.7b00564