NADPH oxidases are important sources of reactive oxygen species (ROS), possibly contributing to various disorders associated with enhanced proliferation. NOX4 appears to be involved in vascular signaling and may contribute to the response to hypoxia. However, the exact mechanisms controlling NOX4 levels under hypoxia are not resolved. We found that hypoxia rapidly enhanced NOX4 mRNA and protein levels in pulmonary artery smooth-muscle cells (PASMCs) as well as in pulmonary vessels from mice exposed to hypoxia. This response was dependent on the hypoxia-inducible transcription factor HIF-1alpha because overexpression of HIF-1alpha increased NOX4 expression, whereas HIF-1alpha depletion prevented this response. Mutation of a putative hypoxia-responsive element in the NOX4 promoter abolished hypoxic and HIF-1alpha-induced activation of the NOX4 promoter. Chromatin immunoprecipitation confirmed HIF-1alpha binding to the NOX4 gene. Induction of NOX4 by HIF-1alpha contributed to maintain ROS levels after hypoxia and hypoxia-induced proliferation of PASMCs. These findings show that NOX4 is a new target gene of HIF-1alpha involved in the response to hypoxia. Together with our previous findings that NOX4 mediates HIF-1alpha induction under normoxia, these data suggest an important role of the signaling axis between NOX4 and HIF-1alpha in various cardiovascular disorders under hypoxic and also nonhypoxic conditions.