Streptococcus agalactiae, also known as group B Streptococcus (GBS), is a Gram-positive encapsulated bacterium that colonizes the gastrointestinal tract of 30 to 50% of humans. GBS causes invasive infection during pregnancy that can lead to chorioamnionitis, funisitis, preterm prelabor rupture of membranes (PPROM), preterm birth, neonatal sepsis, and maternal and fetal demise. Upon infecting the host, GBS encounters sentinel innate immune cells, such as macrophages, within reproductive tissues. Once phagocytosed by macrophages, GBS upregulates the expression of the gene , which encodes an NADH peroxidase. GBS mutants with an deletion (Δ) are exquisitely sensitive to reactive oxygen stress. Furthermore, we have shown that is required for GBS survival in both THP-1 and placental macrophages. In an murine model of ascending GBS vaginal infection during pregnancy, is required for invading reproductive tissues and is critical for inducing disease progression, including PPROM and preterm birth. Reproductive tissue cytokine production was also significantly diminished in Δ mutant-infected animals compared to that in animals infected with wild-type (WT) GBS. Complementation in reversed this phenotype, indicating that is critical for GBS survival and the initiation of proinflammatory signaling in the gravid host. This study sheds new light on the way that group B Streptococcus (GBS) defends itself against oxidative stress in the infected host. The enzyme encoded by the GBS gene is an NADH peroxidase that, our study reveals, provides defense against macrophage-derived reactive oxygen stress and facilitates infections of the uterus during pregnancy. This enzyme could represent a tractable target for future treatment strategies against invasive GBS infections.