Proton nuclear magnetic resonance (NMR) of liquid water can be used to indirectly detect and assay methane hydrate in opaque earth materials. It is also potentially useful for quantifying pore size control of hydrate formation, and for estimating in situ hydraulic permeability of hydrate-affected earth formations. The method is quantitative, nondestructive, and volumetrically averaging over a spatially selected region. In the first controlled validation of this measurement, we have used NMR to measure changes in the liquid water content of sediment and rock resulting from methane hydrate formation. Hydrates were artificially formed at the seafloor by introducing methane into tubes containing sediments or rock saturated with seawater; after several weeks the samples were visited by a remotely operated vehicle on which NMR equipment was mounted. Reduction of the NMR signal indicated hydrate had formed in the sample within a well-defined measurement volume. Hydrate content determined by NMR quantitatively predicted the volume of methane gas evolved during subsequent dissociation.