Information relating to the fracture toughness of geomaterials is critical to our understanding of tensile fracturing, and in particular in geological and rock engineering projects that are subjected to elevated moisture levels. In this study, we conducted a comprehensive set of fracture toughness tests on a suite of key rock types in air under different relative humidities and at constant temperature in order to investigate the influence of relative humidity on fracture toughness. Three sandstones and two igneous rocks were chosen for this purpose. We show that the value of fracture toughness decreases with increasing relative humidity. In addition, we find that the decrease in fracture toughness was more significant when the expansive clay such as smectite was included in rock. Since smectite is prone to expanding in the presence of water, the strength and thus crack growth resistance decrease when relative humidity is high. Therefore, we interpret the decreasing fracture toughness upon the degradation of expansive clays with increasing water content. It was also shown that the decrease of the fracture toughness with increasing humidity is less significant than the concomitant decrease in the measured value of the subcritical stress intensity factor. This was likely as a result of stress corrosion having little influence on the fracture toughness. We conclude that crack growth in rock is affected by humidity, and that clay content is an important contributing factor to changes in fracture toughness and subcritical stress intensity factor.