© 2021 Elsevier B.V.The effect of water temperature on the growth performance of triploid juvenile turbot (Scophthalmus maximus) at sub-optimal water temperatures is unknown. In this study, the effect of constant water temperatures (10, 14, 18, and 22 °C) on growth performance and feed conversion ratio of hatchery reared juvenile diploid and triploid turbot were evaluated for a period of 3 months. No mortality occurred during the experiments. A positive correlation was found between fish growth rate and water temperature when it was increased from 10 to 18 °C. The growth performance of the diploid and triploid groups at 18 °C was higher than those of reared at other temperatures. The growth parameter indices (specific growth rate, and weight and length gain ratio), and the feed conversion ratio of the ploidy groups were significantly different. The lowest growth in length and weight was observed in ploidy groups reared at 10 °C in which the growth rate of the triploid group in weight was significantly lower than the diploid group. Using a parabolic regression model, the optimum temperature for growth (Topt.G) and feed conversion ratio (Topt.FCR) in diploid and triploid turbot were estimated as 18.7–18.9 °C and 15.8–16.8 °C, respectively. The optimum water temperature for the feed conversion ratio was slightly higher in triploids compared to diploid counterparts, whereas the optimum water temperature for growth was relatively similar. The result of this study demonstrates that juvenile triploids grow and feed similarly with their diploid counterparts at different temperatures without any mortalities.