AbstractThis study was carried out to determine the effect of deficit irrigation applications on yield and quality criteria in 20-year-old grapefruit (Citrus paradisi Macf. cv. Star Ruby), irrigated by subsurface drip irrigation method in Batı Akdeniz Agricultural Research Institute between 2018 and 2019. In the research, full irrigation I1 (100%) and deficit irrigation I2 (67%), I3 (33%) treatments were tested. Considering the 2-year averages, irrigation water amounts of the I1, I2, and I3 irrigation levels were 597.9, 407.6, and 217.2 mm, respectively. Evapotranspiration (ET) values were 708.8 mm for I1, 566.3 mm for I2, and 406.3 mm for I3. Total yield was determined as 48.03 t ha−1 for I1, 41.90 t ha−1 for I2, and 36.57 t ha−1 for I3. Fruit weight, fruit width, fruit length, skin thickness, juice yield, and pH decreased in accordance with the water stress imposed. However, the amount of water-soluble dry matter, titratable acidity, total dry matter amount, total flavonoid, total phenol, antioxidant, and ascorbic acid amounts increased when the irrigation level decreased. Although there were no statistical differences between sucrose, glucose, and fructose amounts, the amount of sucrose, glucose, and fructose increased as the irrigation water level decreased in general. The maximum net income was obtained from I1 treatment followed by I2 and I3 irrigation treatments. According to these results, if the subsurface drip irrigation method is used, grapefruit should be irrigated to bring the soil water content to the field capacity, when 30% of the available moisture in the soil is consumed.