A large fraction of atmospheric mineral aerosol is composed of clays, such as smectites. Smectites are known to expand upon addition of water, and thus their properties in the atmosphere may vary strongly with relative humidity (RH). Here we report on the adsorption of water to Na-montmorillonite, a smectite clay. We probe the water uptake under conditions representative of the Earth's troposphere, as well as those relevant for the surface of Mars, of which montmorillonite is a proposed component. Using a vacuum chamber equipped with transmission Fourier transform infrared spectroscopy, we find that Na-montmorillonite contains 10% water by mass at temperatures from 212 K to 231 K at 50% RH. Surprisingly, the water uptake by Na-montmorillonite is almost as great as that of deliquesced ammonium sulfate. We also find that although water adsorption to Na-montmorillonite depends strongly on RH, there is not a strong dependence on absolute temperature. In addition, we find the time required for the clay to become saturated with water decreases with increasing water vapor pressure and is much shorter than suggested in previous studies. We discuss the implications of these results for the Earth's troposphere and the potential role of montmorillonite in the Martian hydrologic cycle.