Climate change is expected to affect the quality of soil and water, resulting in a significant impact on soil water balance in various regions around the world. Soil water potential plays a significant role in influencing evapotranspiration (ET), which is a crucial component of the soil water balance. The matric potential and the osmotic potential are the main components of the soil water potential. The osmotic potential is particularly important in dry soils, salt-affected soils, coastal lands, or when low-quality water is utilized for irrigation. Despite its importance, the impact of osmotic potential on ET has not been well-studied compared to other factors. Therefore, we conducted a study to investigate the impact of osmotic potential on ET from small-scale lab lysimeters planted with grass and equipped with scales and data loggers connected to computers. To create different osmotic potential levels, we irrigated the lysimeters with two different water qualities: distilled water and 4.79 dS.m−1 NaCl solution. The lysimeters were kept in well-watered conditions, and daily ET was monitored. Our results indicate a strong correlation between osmotic potential and ET. After three months of applying the treatments, the lysimeters with lower osmotic potential had a 39% reduction in cumulative ET compared to those irrigated with distilled water. Moreover, the osmotic stress affected plant health, leading to a notable decrease in the leaf area index and exerting a significant influence on evapotranspiration partitioning components, including transpiration and evaporation.