Background Inflammatory idiopathic myositis (IIM) comprises a heterogeneous group of systemic muscular diseases that can occur together with other connective tissue diseases (CTD), named overlap myositis (OM). The question of whether OM is a distinct entity still remains controversial. Aim The present study was conducted to assess the clinical and prognostic differences between patients diagnosed with OM, primary polymyositis (PM) and primary dermatomyositis (DM). Method The study consists of a retrospective longitudinal and multicenter series of IIM patients. Patients were classified as OM, PM and DM. Overlap myositis was defined as patients fulfilling criteria for IIM plus criteria for other CTD (namely systemic sclerosis, systemic lupus erythematosus, mixed connective tissue disease, rheumatoid arthritis and primary Sjögren's syndrome). Result A total of 342 patients were included (98 OM, 137 PM and 107 DM). Overlap myositis patients, in comparison with PM and DM, showed significant differences, with more extramuscular involvement, particularly more arthritis (66%, 34.6% and 48.1%, respectively), puffy fingers (49.5%, 11.1% and 24.3%), sclerodactyly (45.4%, 2.2% and 2%), dysphagia (41.8%, 18.2% and 26.4%), Raynaud phenomenon (65.3%, 16.9% and 19.8%), leucopenia (28.9%, 2.2% and 8.4%), thrombocytopenia (8.2%, 2.2% and 1.9%), interstitial lung disease (ILD) (48%, 35% and 30.8%), renal manifestations (13.4%, 3.7% and 1.9%), and more severe infections (41.3%, 26.7% and 21%). No significant differences were found in survival between groups in log rank test (P = 0.106). Multivariate adjusted survival analyses revealed a worse prognosis for severe infections, ILD and baseline elevation of acute phase reactants. Conclusion Overlap myositis stands out as a distinct entity as compared to PM and DM, featuring more extramuscular involvement and more severe infections. Close monitoring is recommended in this subset for early detection and treatment of possible complications.