The present study was undertaken to evaluate the water status and fruit quality of Satsuma mandarin trees (Citrus unshiu Marc. cv. Okitsu wase) affected by changing the nutrient concentration and grown hydroponically from mid-September to harvest. The water potential of nutrient concentration was kept at -0.08 MPa as the control or gradually decreased to -0.43 MPa as the treatment. After the water potential of nutrient solution was lower than -0.30 MPa, both water potential and osmotic potential of leaves and vesicles of fruit gradually decreased as compared with those of control trees. However, turgor of both tissues was maintained at the same value during the treatment. As fruit growth nearly stopped for 30 days after the water potential of nutrient solution was lower than -0.30 MPa, the fruit size of high nutrient concentration was smaller than that of the control. The content of sucrose, fructose, and glucose of fruit grown under high nutrient concentration were significantly greater than those of the control, but acidity showed no differences in both treatments. The application of hydroponic culture indicated that it is possible to control plant water status, fruit growth, and to increase the sugar content of fruit by an increase in the nutrient concentration.