Self-diffusion coefficients of water molecules (1H2O) in Na-stevensite gel were measured by pulsed-gradient spin-echo (PGSE) proton nuclear magnetic resonance (NMR). The effects of clay fraction (0.00–37.7 wt.%) and temperature (20.0–60.3°C) were studied. The results show: (1) phenomenologically, the H2O self-diffusivity in the clay gel, D, is expressed by D/D0 = exp(−0.0198w) where D0 is the H2O self-diffusivity in bulk water of the temperature and w is the clay weight fraction (wt.%). (2) The activation energy of the diffusivity in the stevensite gel is nearly equal to that in bulk water. Thus, the normalized diffusivity, D/D0, obeys a temperature-independent master curve. (3) The exponential dependence of D/D0 on w for w < 25 wt.% (≈ 12 vol.%) can be explained by a random walk model, in which unbound H2O molecules diffuse in the geometrically tortuous pore structure of randomly scattered clay mineral grains. (4) The measured diffusivity can also be explained by a model of unbound H2O diffusing in a polymer network with a specific mesh-size or characteristic interval of the crosslinkage.