Cardiac event is a major cause of death in patients with idiopathic inflammatory myopathies (IIM). The most frequent IIMs are polymyositis (PM) and dermatomyositis (DM). The purpose of this study was to analyze cardiac involvement by three-dimensional speckle-tracking echocardiography (3D STE) in patients with PM or DM, and to identify the relationship of cardiac injury with clinical characteristics and disease-specific parameters. 60 PM/DM patients with preserved left ventricular ejection fraction and 30 matched healthy controls were assessed by conventional echocardiography, 3D STE with biventricular strain analysis and electrocardiogram. Compared to controls, patients with PM/DM had significantly diminished left ventricular global longitudinal systolic strain and right ventricular longitudinal systolic strain (LVGLS, − 20.3 ± 2.5 vs. − 23.4 ± 1.7%; RVLS, − 19.4 ± 4.2 vs − 24.8 ± 2.0%; both P < 0.001), and longer QTc intervals(421.0 ± 38.4 vs 400.6 ± 14.5 ms, P = 0.001). Multiple regression analysis showed that Myositis Damage Index (MDI) was independently associated with LVGLS (R² = 0.44, P = 0.002) and RVLS (R² = 0.56, P < 0.001) in PM/DM patients with established disease course more than 1 year. In multivariate analysis of pooled data for all the PM/DM patients, when MDI was excluded due to missing observations, disease duration correlated with worse LVGLS (R² = 0.24, P = 0.002), while concomitant interstitial lung disease correlated with worse RVLS (R² = 0.30, P < 0.001). Disease activity scores (Myositis Intention to Treat Activities Index) had a weak positive correlation with QTc intervals (r_sp = 0.31, P = 0.02). Our results suggest that cardiac injury in PM/DM is a long-term process and its severity depends on patients’ heterogeneous clinical features and systemic disease burden.