Aims: To characterise the idiopathic inflammatory myopathies (IIM) module of the Rheumatic Diseases Portuguese Register (Reuma.pt/myositis) and the patients in its cohort. Methods: Reuma.pt is a web-based system with standardised patient files gathered in a registry. This was a multicentre open cohort study, including patients registered in Reuma.pt/myositis up to January 2022. Results: Reuma.pt/myositis was designed to record all relevant data in clinical practice and includes disease-specific diagnosis and classification criteria, clinical manifestations, immunological data, and disease activity scores. Two hundred eighty patients were included, 71.4% female, 89.4% Caucasian, with a median age at diagnosis and disease duration of 48.9 (33.6-59.3) and 5.3 (3.0-9.8) years. Patients were classified as having definite (N=57/118, 48.3%), likely (N=23/118, 19.5%), or possible (N=2/118, 1.7%) IIM by 2017 EULAR/ACR criteria. The most common disease subtypes were dermatomyositis (DM, N=122/280, 43.6%), polymyositis (N=59/280, 21.1%), and myositis in overlap syndromes (N=41/280, 14.6%). The most common symptoms were proximal muscle weakness (N=180/215, 83.7%) and arthralgia (N=127/249, 52.9%), and the most common clinical signs were Gottron’s sign (N=75/184, 40.8%) and heliotrope rash (N=101/252, 40.1%). Organ involvement included lung (N=78/230, 33.9%) and heart (N=11/229, 4.8%) involvements. Most patients expressed myositis-specific (MSA, N=158/242, 65.3%) or myositis-associated (MAA, 112/242, 46.3%) antibodies. The most frequent were anti-SSA/SSB (N=70/231, 30.3%), anti-Jo1 (N=56/236, 23.7%), and anti-Mi2 (N=31/212, 14.6%). Most patients had a myopathic pattern on electromyogram (N=101/138, 73.2%), muscle oedema in magnetic resonance (N=33/62, 53.2%), and high CK (N=154/200, 55.0%) and aldolase levels (N=74/135, 54.8%). Cancer was found in 11/127 patients (8.7%), most commonly breast cancer (N=3/11, 27.3%). Most patients with cancer-associated myositis had DM (N=8/11, 72.7%) and expressed MSA (N=6/11) and/or MAA (N=3/11). The most used drugs were glucocorticoids (N=201/280, 71.8%), methotrexate (N=117/280, 41.8%), hydroxychloroquine (N=87/280, 31.1%), azathioprine (N=85/280, 30.4%), and mycophenolate mofetil (N=56/280, 20.0%). At the last follow-up, there was a median MMT8 of 150 (142-150), modified DAS skin of 0 (0-1), global VAS of 10 (0-50) mm, and HAQ of 0.125 (0.000-1.125). Conclusions: Reuma.pt/myositis adequately captures the main features of inflammatory myopathies’ patients, depicting, in this first report, a heterogeneous population with frequent muscle, joint, skin, and lung involvements.