Our aim was to determine the effects of a citrus by-product (CBP) and CBP fermented by Lactobacillus plantarum (LP-CBP), provided as dietary supplements, on the growth performance, feed utilization, innate immunity and temperature tolerance of red seabream. A diet without inclusion of CBP or LP-CBP was used as a control and four other experimental diets were formulated to replace wheat flour by 4% and 8% of either CBP or LP-CBP (designated as Con, LP-CBP4%, LP-CBP8%, CBP4% and CBP8%, respectively). Experimental diets were fed to triplicate groups of 25 fish (initial body weight, 55.0 g) for 9 weeks. Growth performance and feed utilization were not significantly different among all the groups. Bone collagen content was significantly increased by supplementation with CBP and LP-CBP. Vitamin C concentration tended to be higher in livers of fish fed the supplements than in the control group. Myeloperoxidase, lysozyme and superoxide dismutase activities were higher in fish fed CBP or LP-CBP than in fish fed the control diet. When fish were exposed to low water temperature, cumulative mortalities of those fed CBP or LP-CBP supplemented diets were lower (29%, 33%, 34% and 33% mortalities for LP-CBP4%, LP-CBP8%, CBP4% and CBP8%, respectively) than in the control group (58%). Therefore, inclusion of either CBP or LP-CBP at up to 8% in red seabream diet brings benefits through enhanced innate immunity and better tolerance of low water temperature.