BMAL1 (brain and muscle ARNT-like protein 1, also known as MOP3 or ARNT3) belongs to the family of the basic helix–loop–helix (bHLH)-PAS domain-containing transcription factors, and is a key component of the molecular oscillator that generates circadian rhythms. Here, we report that BMAL1-deficient cells are significantly more susceptible to infection by two major respiratory viruses of the Paramyxoviridae family, namely RSV and PIV3. Embryonic fibroblasts from Bmal1^–/– mice produced nearly 10-fold more progeny virus than their wild type controls. These results were supported by animal studies whereby pulmonary infection of RSV produced a more severe disease and morbidity in Bmal1^–/–mice. These results show that BMAL1 can regulate cellular innate immunity against specific RNA viruses.