Abstract Estriol (E3) is an endogenous estrogen in females with broad biological activity within diverse tissue types. In the context of certain T-cell–mediated autoimmune inflammatory diseases, E3 can ameliorate disease severity through immunomodulatory mechanisms that decrease tissue inflammation. Severe disease caused by influenza A virus (IAV) infection is also characterized by aberrant inflammation and immunopathology. How E3 might affect the pathogenesis of IAV infection, however, has not been explored. Gonadally intact female C57BL/6 mice that were treated with exogenous E3 during infection with mouse-adapted 2009 H1N1 had reduced total pulmonary inflammation and improved disease outcomes compared with females that received no hormone. Furthermore, compared with no hormone treatment, E3 treatment reduced the induction of genes associated with proinflammatory cytokine and chemokine responses in the lungs, which preceded clinical disease, reductions in innate immune cell recruitment, altered pulmonary T-cell skewing, and reduced antibody titers during IAV infection. Although E3 treatment was associated with reduced local and systemic anti-influenza adaptive immune responses, there was no effect of E3 on viral replication or clearance. Together, these data suggest that exogenous E3 confers protection during IAV infection through immunomodulatory mechanisms and that E3 may have broad therapeutic potential in the context of both infectious and noninfectious inflammatory diseases. Treatment of female C57BL/6 mice with exogenous estriol during IAV infection decreased lung inflammation but not virus control or clearance, to improve clinical disease outcome.