This study was conducted to determine crop water stress index (CWSI) values and irrigation timing in the case of Derinkuyu dry bean ( Phaseolus vulgaris L.). In 2017, dry beans were grown as the main crop according to the field design consisting of plots divided into randomised blocks. Irrigation treatment comprised full irrigation (I100) and irrigation issues with three different levels of water stress (I66, I33, I0). This study applied 602 mm of water under the I100 irrigation. The yield of Derinkuyu dry beans was equal to 3576.6 kg∙ha ^–1 in I100 irrigation. The lower limit (LL) value, which is not necessary for the determination of CWSI, was obtained as the canopy–air temperature difference ( Tc – Ta) versus the air vapour pressure deficit ( VPD). The upper limit (UL) value, at which the dry beans were wholly exposed to water stress, was obtained at a constant temperature. The threshold CWSI value at which the grain yield of dry beans started to decrease was determined as 0.33 from the measurements made with an infrared thermometer before irrigation in I66 irrigation treatment. As a result, it can be suggested that irrigation should be applied when the CWSI value is 0.33 in dry beans. Furthermore, the correlation analysis revealed a negative correlation between grain yield and crop water stress index and a positive correlation between yield and chlorophyll content. According to variance analysis, significant relationships were found between the analysed parameters at p ≤ 0.01 and p ≤ 0.05.