Preterm birth (PTB), defined as birth occurring before 37 weeks of pregnancy, affects 5–18% of pregnancies and is the leading cause of neonatal morbidity and mortality worldwide. Although PTB is considered a syndrome, infection-induced inflammation accounts for up to 50% of all cases. Despite the effort to reduce the incidence of PTB, it continues to rise worldwide and current approaches for preventing or treating PTB are largely unsatisfactory. Probiotics are live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. It is well known that probiotics can modulate the host immune system exerting a potent anti-inflammatory activity. The main aim of this work was to evaluate the capacity of the probiotic Lactobacillus kefiri (Lk48) to prevent preterm birth in mice. C57BL/6 female mice were treated with Lk48 or vehicle a week before and during pregnancy and were challenged with LPS (10 µg), a dose known to induce PTB on gestational day 16. Percentages of PTB as well as stillbirth were evaluated. We observed that oral administration of Lk48 significantly reduced the occurrence of LPS-induced PTB and stillbirth as well as improved post-natal development. This protective effect was associated with a reduction in leucocyte infiltration and reduced inflammation-induced damage in reproductive tissue. Besides, Lk48 treatment also modulated the diversity of vaginal microbiota. Our results demonstrated that prophylactic consumption of probiotic L. kefiri prevented LPS-induced PTB and still birth in mice and opens new avenues for exploring novel and promising strategies for preventing PTB in humans.