The objective of this study was to evaluate the sodium butyrate in the feeding of pirarucu juveniles, Arapaima gigas. Seventy - five juveniles of A. gigas (132.07 ± 3.12 g) were distributed in fifteen 310 L- tanks with constant aeration, filtration system and closed water recirculation. A completely randomized design the experiment, with four treatments including sodium butyrate (98%) in the diets (BS0 = control diet without sodium butyrate, BS1 = 0.5 g kg−1, BS2 = 1.0 g kg−1, BS3 = 1.5 g kg−1, BS4 = 2.0 g kg−1) with three replicates, and five juvenile treatment−1. The fish were fed three times day−1 (8, 12 and 16 h), until apparent satiety, for 45 days. Water quality parameters such as temperature, pH, dissolved oxygen and ammonia were monitored three times in the week. The growth parameters as weight gain, mean feed consumption, feed conversion ratio, specific growth rate, and survival were evaluated. It was performed chemical analysis of the whole body to establish dry matter, crude protein, ethereal extract, ash and gross energy. The hepatosomatic index and viscerosomatic index of juveniles also were evaluated. Histological sections of the intestine showed greater villous height and enzymatic activity of amylase, lipase and nonspecific alkaline protease. The blood plasma of juveniles was analyzed, as the hematocrit, total plasma protein concentration, triglyceride, glucose, and cholesterol. The inclusion of 1.17 g kg−1 of sodium butyrate in diets for A. gigas juveniles improves the growth parameters. This level favors feed conversion ratio increased intestinal villous height, without interfering in the blood parameters, and promote the adaptation of the enzymatic activity of amylase, lipase and nonspecific alkaline protease. Furthermore, it reduces the activity of aspartate aminotransferase and increases hepatic glycogen levels, making the animals more resistant to potentially stressful conditions in the growing environment. •The additive sodium butyrate, up to 1.17 g kg−1, improves the growth parameters of piraruru juveniles (Arapaima gigas).•Sodium butyrate decreased apparent feed conversion of pirarucu juveniles.•The use of this additive increases the villus height of the anterior and middle intestine.•The use of sodium butyrate probably acts on the health of pirarucu, increasing the reserve glycogen.