There is growing awareness that brain mechanical properties are important for neural development and health. Yet, published values of brain stiffness differ by orders of magnitude from ex vivo to in vivo, pointing to a general lack of understanding of the complex mechanical behavior of brain tissue. We here show that there is no fundamental disparity between ex vivo and in vivo data when considering large-scale properties of the entire brain. Using numerical simulations and novel real-time magnetic resonance elastography we investigated the viscoelastic dispersion of the human brain in, so far, unexplored dynamic ranges from intrinsic brain pulsations at 1Hz to externally induced harmonic vibrations at 40Hz. Surprisingly, we observed variations in brain stiffness over more than two orders of magnitude, suggesting that the in vivo human brain is superviscous on large scales with very low shear modulus of 42±13 Pa and relatively high viscosity of 6.6±0.3 Pa∙s according to the two-parameter solid model. Our data shed light on the crucial role of fluid compartments including blood vessels and cerebrospinal fluid (CSF) for whole brain properties and provide, for the first time, an explanation for the variability of the mechanical brain responses to manual palpation, local indentation, and high-dynamic tissue stimulation as used in elastography.