PURPOSE: To determine whether oral probiotic administration (1.5 × 10 CFU/g Lacticaseibacillus paracasei subsp. paracasei) to breastfeeding mothers or to their very low birth weight (VLBW) infants reduces total serum bilirubin (TSB) level and increases weight gain of the infants. METHODS: In this double-blind trial, breastfeeding mothers and their VLBW infants at 48-72 h of age were allocated into three groups using stratified block randomization; administrating probiotic to the mothers and placebo to their infants, probiotic to the infants and placebo to their mothers, or placebo to the both. The intervention continued for 28 days. RESULTS: All 25 mothers and their 26 infants allocated into each group were fully followed up. There were three positive blood culture only in the placebo group. On the 3rd day of intervention, TSB level was not significantly different among the groups but on the 7th day, it was significantly lower in the both probiotic groups compared with the placebo group (mean difference  -2.4 mg/dL [95% confidence interval  -3.6 to  -1.2] in the both comparisons). Mean rank of infant weight gain during the first 7 days of intervention was significantly higher in the both maternal and infant probiotic groups compared with the placebo group (p = 0.007 and p = 0.003, respectively), but there was no statistically significant difference among the groups during the 8th-28th days. CONCLUSION: Administration of Lacticaseibacillus paracasei to breastfeeding mothers of VLBW infants or to their infants reduces infant TSB level but has no significant effect on weight gain after the first week of the intervention. TRIAL REGISTRATION: The Iranian Clinical Trials Registry IRCT20100414003706N38. protrospectively registered 24/01/2021.