The longitudinal NMR relaxation rate (R1) of water protons was measured on aqueous suspensions of Gd@BNNTs (i.e., boron nitride nanotubes doped with 0.4% w/w of Gd) coated with glycol-chitosan in the 0.01–30.0 MHz Larmor frequency range, by using a fast field-cycling relaxometer. The dispersion curve of the relaxivity (r1) relative to Gd, determined exploiting R1 data from Gd@BNNT and BNNT suspensions, shows a logarithmic dependence on frequency which is characteristic of a relaxation enhancement dominated by an outer sphere mechanism governed by two-dimensional diffusion of water in proximity of Gd3+ ions on the BNNT surface. Quantitative information on water diffusion and affinity for the surface was obtained by analyzing the r1 NMRD curves in terms of a model for two-dimensional diffusion. The effects of the nanotube assembly in water and of the coating hydrophilicity on the interactions between water molecules and Gd3+ ions governing the proton relaxation enhancement at the basis of contrast in magnetic resonance imaging (MRI) are discussed.