In the nanometer length-scale, structural stability is prone to ultrafast motions that range from atomic vibrations to translations and rotations of entire nanostructures. To study the rotational motions of entire nanoparticles, we establish femtosecond electron diffraction as goniometer of ultrafast nanocrystal rotations. To achieve our scope we have combined experiments that use ultrashort pulses of electrons and photons, with detailed simulations of molecular dynamics and electron diffraction.