tLeptospirosis is a zoonotic disease and is caused by pathogenic species of the Leptospira genus, includingLeptospira interrogans (L. interrogans). Humans, domestic and wild animals are susceptible to acute orchronic infection. The innate immune response is a critical defense mechanism against Leptospira inter-rogans, and has been investigated in mouse models. Murine Toll-like receptors (TLRs) have been shownto be key factors in sensing and responding to L. interrogans infection. Specifically, TLR2, TLR4 and theTLR adaptor molecule MyD88 are essential for host defense against L. interrogans; however, the role ofthe TLR adaptor molecule TIR-domain-containing adaptor-inducing interferon (TRIF) in the responseto L. interrogans has not been previously determined. In the present study, TRIF was found to play animportant role during leptospiral infection. Following challenge with L. interrogans, Trif−/−mice exhib-ited delayed weight gain compared to wild-type mice. Moreover, Trif−/−mice exhibited an increase in L.interrogans burden in the kidneys, lungs, and blood at early time points (less than 7 days post infection).Multiple components of the innate immune responses were dampened in response to leptospiral infec-tion including transcription and production of cytokines, and the humoral response, which suggestedthat TRIF contributes to expression and production of cytokines important for the host defense againstL. interrogans.