Abstract Objectives The aim of this study was to investigate cardiac involvement detected by ECG in patients with idiopathic inflammatory myopathies (IIMs) and to evaluate possible associations between the autoantibody profile and ECG changes in these patients. Methods In a Scandinavian cross-sectional study, patients were included from two Danish centres and one Swedish centre. Resting 12-lead ECG was investigated in 261 patients with IIM compared with 102 patients with systemic sclerosis (SSc) and 48 healthy controls (HCs). ECG changes were correlated to clinical manifestations and myositis-specific and myositis-associated autoantibodies (MSAs and MAAs, respectively). Results Patients with IIM had a longer mean corrected QT (QTc) duration and more frequently presented with prolonged QTc (≥450 ms; P = 0.038) compared with HCs. A longer QTc duration was recorded in SSc compared with IIM [433 ms (s.d. 23) vs 426 (24); P = 0.011], yet there was no significant difference in the fraction with prolonged QTc (SSc: 22%, IIM: 16%; P = 0.19). In multivariable regression analyses, anti-Mi2 (P = 0.01, P = 0.035) and anti-Pl-7 (P = 0.045, P = 0.014) were associated with QTc duration and prolonged QTc in IIM. Elevated CRP was associated with prolonged QTc (P = 0.041). Conclusion The presence of QTc abnormalities was as common in patients with IIM as in patients with SSc, including prolonged QTc seen in almost one-fifth of the patients. Anti-Mi2, anti-Pl-7 and elevated CRP may serve as biomarkers for cardiac disease in IIM, but needs to be confirmed in a larger prospective study.