Collectins are C-type lectins that are involved in innate immunity as pattern recognition molecules. Recently, collectin kidney 1 (CL-K1) has been discovered, and in vitro studies have shown that CL-K1 binds to microbes and activates the lectin complement pathway. However, in vivo functions of CL-K1 against microbes have not been elucidated. To investigate the biological functions of CL-K1, we generated CL-K1 knockout (CL-K1-/-) mice and then performed a Streptococcus pneumoniae infection analysis. First, we found that recombinant human CL-K1 bound to S. pneumoniae in a calcium-dependent manner, and induced complement activation. CL-K1-/-mice sera formed less C3 deposition on S. pneumoniae. Furthermore, immunofluorescence analysis in the wild-type (WT) mice demonstrated that CL-K1 and C3 were localized on S. pneumoniae in infected lungs. CL-K1-/-mice revealed decreased phagocytosis of S. pneumoniae. Consequently, less S. pneumoniae clearance was observed in their lungs. CL-K1-/-mice showed severe pulmonary inflammation and weight loss in comparison with WT mice. Finally, the decreased clearance and severe pulmonary inflammation caused by S. pneumoniae infection might cause higher CL-K1-/-mice lethality. Our results suggest that CL-K1 might play an important role in host protection against S. pneumoniae infection through the activation of the lectin complement pathway.