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Objective Patients with anti‐melanoma‐differentiation‐associated 5 (anti‐MDA5)‐positive dermatomyositis (DM) share several striking similarities to patients with SARS‐CoV‐2. Our objective was to assess the prevalence of anti‐angiotensin converting enzyme‐2 (ACE2) immunoglobulin M (IgM) antibodies, found in patients with severe SARS‐CoV‐2, in two independent anti‐MDA5‐positive DM cohorts. Methods Anti‐ACE2 IgM antibodies were assayed by enzyme‐linked immunosorbent assay (ELISA) in two anti‐MDA5‐positive DM cohorts: a predominantly outpatient North American cohort (n = 52) and a Japanese cohort enriched for new‐onset disease (n = 32). Additionally, 118 patients with SARS‐CoV‐2 with a spectrum of clinical severity were tested for anti‐MDA5 antibodies by ELISA. Results Five of fifty‐two (9.6%) North American patients and five of thirty‐two (15%) Japanese patients were positive for anti‐ACE2 IgM. In the North American cohort, all five patients with anti‐ACE2 IgM antibodies had proximal muscle weakness, had interstitial lung disease, were significantly more likely to receive pulse dose methylprednisolone (80% vs 30%, P = 0.043), and had worse forced vital capacity (median 59% predicted vs 78%, P = 0.056) compared with the anti‐ACE2‐IgM‐negative group. In the Japanese cohort, all five anti‐ACE2‐IgM‐positive patients also required pulse dose methylprednisolone, and three of five (60%) patients died. Japanese patients with anti‐ACE2 IgM had significantly worse oxygenation, as defined by a lower partial pressure of oxygen (PaO2)/fraction of inspired oxygen (FiO2) ratio (233 vs 390, P = 0.021), and a higher alveolar‐arterial oxygenation gradient (91 vs 23 mm Hg, P = 0.024) than the IgM‐negative group. Conclusion We describe anti‐ACE2 IgM autoantibodies in two independent cohorts with anti‐MDA5‐positive DM. These autoantibodies may be biomarkers for severe disease and provide insight into disease pathogenesis.