Electrospinning is used to deform originally spherical polymer nanoparticles into ellipsoidal nanoparticles. The polymer nanoparticles are swollen and the dispersion is then electrospun. Under certain conditions, the stretching generated in the electrospinning jet is enough to generate elongated nanoparticles embedded in fibers. The formation of the anisotropic particles is observed by stimulated emission depletion (STED) microscopy performed on fluorescent nanoparticles and by electron microscopy measurements on the nanoparticles recovered after removal of the fiber matrix.