We report on the direct observation of the intrinsic magnetization behavior of Mackay icosahedral Au nanoparticles. The spin arrangements in 3.5-nm icosahedral Au particles are found to be ferrimagnetic-like, where the core and surface moments point in opposite directions, with a net spontaneous magnetic moment of 16 mu(B) per particle. The unpaired spins behave as J = 1/2 quantum spins. The average level separation is found to be a factor of similar to 1.53 larger than that estimated according to the Kubo formula for spherical Au particles. This reflects the fact that there are considerably fewer atoms packed in a particle with an icosahedral geometry than with a spherical one.