Cardiac ischemia is associated with arrhythmias, but effective therapies are limited. The cardiac voltage gated sodium channel α-subunit (SCN5A), encoding the Nav1.5 current, plays a key role in the cardiac electrical conduction and arrhythmic risk. Here, we show that hypoxia reduces Nav1.5 through effects on a microRNA (miR), miR-448. The expression of miR-448 is increased in ischemic cardiomyopathy. miR-448 has a conserved binding site in 3′-UTR of SCN5A. miR-448 binding to this site suppressed SCN5A expression and sodium currents. Hypoxia-induced HIF1α and NF-κB were major transcriptional regulators for MIR448. Hypoxia also relieved MIR448 transcriptional suppression by RE1 silencing transcription factor (REST). Inhibition of miR-448 reduced arrhythmic risk after myocardial infarction. These results indicated that ischemia drove miR-448 expression, reduced Nav1.5 current and increased arrhythmic risk. Arrhythmic risk was improved by preventing Nav1.5 downregulation, suggesting a new approach to antiarrhythmic therapy.