Dietary supplementation with tributyrin (TBT), a source of butyric acid, has been shown to improve growth performance and health status when high levels of plant ingredients are included in aquafeeds. Here, we investigated the response of rainbow trout (Oncorhynchus mykiss) to a plant-based diet supplemented with TBT aiming to improve fish performance and welfare. Juvenile rainbow trout (19.0 ± 0.19 g, mean ± SEM) were divided into 12 tanks (210 L, 40 fish/tank) connected to a RAS system and fed four experimental diets in triplicate to apparent satiety. Diets tested comprised a basal diet (CTR) including 10% fishmeal and 10% fish oil, plus 83% plant-derived ingredients (44% crude protein, 18% crude fat, gross energy 21.8 MJ kg − 1) with increasing inclusion levels of TBT (55% purity), supplying 0.05% (TBT1), 0.1% (TBT2) and 0.2% (TBT4) of this compound. Fish performance was evaluated after 44 days. Following the growth trial, the effect of the diets on selected plasma innate immune and hepatic antioxidant parameters of trout was investigated at 3 h and 24 h after feeding. TBT supplementation had no significant impact on growth performance, feed efficiency, feed intake, or proximal composition in rainbow trout (P > 0.05). However, the plasma lysozyme activity was higher in fish fed the TBT2 diet than in those fed CTR or TBT4 diets after 3 h (P = 0.021). In addition, the plasma antiprotease activity in fish fed the TBT2 and TBT4 diets was higher than those fed the CTR diet after 24 h (P = 0.010 and 0.013, respectively). Catalase activity in the liver of trout fed the TBT4 diet was higher than in fish fed CTR or TBT2 diets after 3 h (P = 0.016 and 0.019). Results showed that effective levels of dietary TBT supplementation at 0.1% and 0.2% could result in improved immune and antioxidant responses in trout, respectively. Therefore, dietary TBT supplementation in this carnivorous species may result in enhanced welfare when high levels of plant-derived ingredients are used in aquafeeds.