Bacillary dysentery is a major cause of children's admission to hospitals. To assess the probiotic and prebiotic (synbiotics) effects in children with dysentery in a randomized clinical trial, 200 children with dysentery were studied in 2 groups: the synbiotic group received 1 tablet/day of synbiotic for 3-5 days and the placebo group received placebo tablets (identical tablet form like probiotics). The standard treatment was administered for all patients. Duration of hospitalization, dysentery, fever, and the weight loss were assessed in each group. It was concluded that there was no significant difference in both groups in the baseline characteristics. The mean duration of dysentery reduced (P < 0.05). The mean duration of fever has been significantly reduced in the synbiotic group (1.64 ± 0.87 days) in comparison to the placebo group (2.13 ± 0.94 days) (P < 0.001). Average amount of weight loss was significantly lower in the synbiotic group in comparison to that in the placebo group (129.5 ± 23.388 grams and 278 ± 28.385 grams, resp.; P < 0.001). There was no significant difference in the mean duration of hospitalization in both groups (P > 0.05). The use of synbiotics as an adjuvant therapy to the standard treatment of dysentery significantly reduces the duration of dysentery, fever, and rate of weight losses. The trial is registered with IRCT201109267647N1.