To properly define stress states controlling the mechanical and hydraulic behavior of unsaturated soil, osmotic and matric suction should be determined, since they affect soil behavior differently. A novel and simple method is presented for determining osmotic suction in soil using a device that measures pore air relative humidity. The method is developed using a chilled mirror device (CMD) and validated by conducting a series of experiments on three different soils mixed with aqueous solutions with various concentrations of different salts. The proposed method involves determining the water content of an unsaturated sample, mixing it with pure water to achieve saturation, and then determining the total suction and water content at the saturated state. At saturation, the measured total suction is equal to the osmotic suction in the sample; thus, by using the water content and osmotic suction for the saturated sample, the osmotic suction corresponding to the water content in the unsaturated condition can be determined. Also, by determining the total suction of the unsaturated sample before mixing with water, matric suction can be computed. While this method extends the usefulness of the CMD, there are limitations associated with the measurement accuracy of the device and solubility limit of salt in the pore water that are discussed in this article.