This study examines the evapotranspiration and crop coefficient of greenhouse-grown bell peppers located in Varamin, Iran, using the weighing lysimeter based on water balance. Additionally, the results were compared with the FAO-Penman-Monteith and Droogers-Allen methods. The crop coefficients were also estimated for different growth stages of bell peppers, and the findings were evaluated against previous studies. Results indicated that the total evapotranspiration during the spring cultivation season for bell pepper and the reference crop was 3.53 mm/day and 5.58 mm/day, respectively. The annual evapotranspiration rate of bell peppers under greenhouse conditions was also estimated to be 761 mm, equivalent to 7612 m3/ha. The reference evapotranspiration calculated using the FAO-Penman-Monteith and Droogers-Allen methods inside the greenhouse was 5.15 mm/day and 4.41 mm/day, respectively, for the entire growth period. Furthermore, the average crop coefficients for the initial, middle, and final growth stages were determined as 0.13, 0.87, and 0.6, respectively. The highest and lowest crop coefficients were observed in the no-fog and no-cover conditions, and in the fog and cover conditions, respectively. Meanwhile, the average annual yield was 166 tons/hectare in this greenhouse. Given the scarcity of prior studies on greenhouse-grown bell peppers under hydroponic conditions, the findings of this research provide a foundation for water conservation, irrigation optimization, and effective irrigation planning.