Considering the increasing rate of antibiotic resistance and consequently the need for using alternative compounds to increase immunity and prevent diseases, the present study aimed to investigate the effects of Lacticaseibacillus casei on the growth indices and intestinal morphology of adult zebrafish. This study was conducted on 80 zebrafish (mean weight: 0.25±0.05 g and mean length: 2.5±0.05 cm), which were assigned to four groups with two replications. Three of the groups received L. casei at each of the concentrations of 1.5×10⁴, 1.5×10⁷, 1.5×10⁸ CFU/ml, and one served as the control, which was fed with the basic diet. Samples were collected to examine the weight (W), length (TL), condition factor (CF), and intestinal morphological changes of the fish at the end of the study. The results showed that the probiotic diet boosted the weight and length of the fish compared to the control group (p>0.05). Based on these results, feeding with L. casei probiotic exerted the most potent and least impact on the growth of the fish at the concentrations of 1.5×10⁸ CFU/ml and 1.5×10⁷ CFU/ml, respectively. There was also a significant change in intestinal villous length after receiving the probiotic diet compared with the control group (p<0.05). However, intestinal villous length was not significantly different comparing the groups receiving different concentrations of the probiotic (p>0.05). Based on the results of this study and observed increased length of intestinal villous after being fed with L. casei, which will subsequently increase the nutrient absorption and growth of fish, it is recommended to use this probiotic at the indicated concentrations (1.5×10⁸ CFU/ml) as a dietary supplement.