<b><i>Objective:</i></b> Nuclear factor kappa-B (NF-&#954;B)-induced inflammation leads to myocarditis and heart dysfunction. How microRNAs (miRNAs) contribute to this process is poorly defined. The aim of this study was to investigate whether miRNAs regulate NF-&#954;B-induced inflammation in experimental autoimmune myocarditis (EAM) in vivo. <b><i>Methods and Results:</i></b> NF-&#954;B and its related proinflammatory genes, including interleukin-6 (IL-6) and tumor necrosis factor-a (TNF-a), were activated in EAM. Profiling of NF-&#954;B-related miRNAs revealed that miR-590-3p was strikingly reduced in EAM. We found IL-6-induced proinflammatory signaling via miR-590-3p reduction, p50 induction, NF-&#954;B activation and IL-6/TNF-a expression. Moreover, a luciferase reporter assay demonstrated that miR-590-3p directly interacted with the 3' UTR (untranslated region) of the p50 subunit, and that miR-590-3p overexpression inhibited p50 expression. Finally, miR-590-3p transfection through adeno-associated virus significantly inhibited p50 expression, suppressed NF-&#954;B activity and blocked IL-6/TNF-a expression in vivo, reducing the lesion area and improving cardiac function in EAM. <b><i>Conclusion:</i></b> miR-590-3p is a novel NF-&#954;B-related miRNA that directly targets the p50 subunit. This may provide a novel strategy for the treatment of myocarditis.