Natural killer (NK) cells are naturally circulating innate lymphocytes that sense altered cells, including pathogen activated and early transformed cells. The signals that prime the NK cell to respond are not completely understood, but cytokines such as IL-12, IL-18, and type I interferon (IFN-αβ) from antigen presenting cells, are appreciated to be key to NK cell effector functions in response to bacteria, viruses and tumours. In this light, a new class of IFN, IFN type III (IFN-λ) has been described, by sharing common functions with IFN-αβ but with a more restricted cellular expression. Here our goal was to demonstrate the ability of IFN-λ to directly regulate NK cell effector functions in vivo, alone and in the context of IFN-αβ, by using gene-targeted mice lacking the IFN-λ receptor (IL-28Rα-/-). Although the deletion of this receptor did not alter the NK cell receptor repertoire, maturation status, in vitro killing of target cells, or in vitro IFN-γ production upon specific stimulation (IFN type I, IL-12, IL-15 and IL-18); the loss of IL-28Rα greatly limited their in vivo IFN-γ production in response to LPS. As consequence IL-28Rα-/- mice showed increased resistance to lethal endotoxicosis and enhanced survival. The limitation in NK cell response was also observed in a cecal ligation and puncture (CLP)-induced septic shock model. Lastly, the loss of IL-28Rα greatly limited tumour control in several models where growth and metastases are NK cell-dependent. This study demonstrates for the first time, the ability of IFN-λ to directly regulate NK cell effector functions in vivo, alone and in the context of IFN-αβ signalling.