Objective: The objective of this study is to evaluate the preventive effect of oral administration of Lactobacillus fermentum CECT5716 on mastitis incidence in lactating women.Methods: A randomized double-blinded controlled trial that included 625 women was conducted. Women who received preventive dose of antibiotic in the context of delivery were recruited 1–6 days after childbirth and randomly assigned to a group. Probiotic group received 1 capsule/day containing L. fermentum 3 × 109 CFU, control group received 1 placebo capsule/day containing maltodextrin. The intervention period was 16 weeks. The primary outcome of the study was the incidence of clinical mastitis defined as at least two out of the three breast symptoms (pain, redness, and lump) and at least one of fever or flu-like symptoms (shivering, hot sweats, or aches).Results: Two hundred ninety-one women completed 16 weeks of treatment. Sixteen women in the probiotic group developed mastitis versus 30 women in the control group (odds ratio = 0.531; p = 0.058). Incidence rate of mastitis in the probiotic group was significantly lower than that in the control group (IR = 0.130 in the probiotic group versus IR = 0.263 in the control group; p = 0.021). Therefore, the oral administration of L. fermentum CECT5716 during lactation decreased by 51% the incidence rate of clinical mastitis. Staphylococcus spp. load at the end of intervention was significantly lower in breast milk of women in the probiotic group than in breast milk of women in the control group (p = 0.025).Conclusion: Consumption of the probiotic strain L. fermentum CECT5716 might be used during breastfeeding as an efficient strategy to prevent development of lactational mastitis in women.Trial registration: NCT02203877.