The average mass density profile measured in the Canadian Network for Observational Cosmology cluster survey is well described with the analytic form ρ(r)= Ar-1 (r+ a ρ)-2, as advocated on the basis of n-body simulations by Navarro, Frenk, & White. The predicted core radii are a ρ= 0.20 (in units of the radius where the mean interior density is 200 times the critical density) for an Ω= 0.2 open cold dark matter model and a ρ= 0.26 for a flat Ω= 0.2 model, with little dependence on other cosmological parameters for simulations normalized to the observed cluster abundance. The dynamically derived local mass-to-light ratio, which has little radial variation, converts the observed light profile to a mass profile. We find that the scale radius of the mass distribution, 0.20≤ a ρ≤ 0.30 (depending on modeling details, with a 95% confidence range of 0.12–0.50), is completely consistent with the predicted values. Moreover, the profiles and total masses of the clusters as individuals can be acceptably predicted from the cluster rms line-of-sight velocity dispersion alone. This is strong support for the hierarchical clustering theory for the formation of galaxy clusters in a cool, collisionless, dark-matter–dominated universe.