Skin, the first barrier against invading microorganisms, is hypoxic, even under baseline conditions. The transcription factor hypoxia-inducible factor-1α (HIF-1α, the principal regulator of cellular adaptation to low oxygen, is strongly expressed in skin epithelium. HIF-1α is now understood to play a key role in the bactericidal capacity of phagocytic cells such as macrophages and neutrophils. In the skin, keratinocytes provide a direct antibacterial activity through production of antimicrobial peptides, including cathelicidin. Here, we generate mice with a keratinocyte-specific deletion of HIF-1α and examine effects on intrinsic skin immunity. Keratinocyte HIF-1α is seen to provide protection against necrotic skin lesions produced by the pathogen group A Streptococcus. RNA interference studies reveal that HIF-1α regulation of keratinocyte cathelicidin production is critical to their antibacterial function.