We present a new class of force sensor based on Brillouin scattering in an optical nanofiber. The sensor is a silica nanofiber of a few centimeters with a submicron transverse dimension. This extreme form factor enables one to measure forces ranging from 10 μN to 0.2N. The linearity of the sensor can be ensured using the multimode character of the Brillouin spectrum in optical nanofibers. We also demonstrated non-static operation and a competitive signal-to-noise ratio as compared to commercial force sensor resistor.