This study aimed to evaluate physical activity (PA) monitoring using wrist-worn accelerometers in the assessment of myositis patients. Included patients had a diagnosis of dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM) or overlap myositis (OM) and were evaluated at baseline (M0) and 6 months (M6). IMACS core-set measures, ACR/EULAR improvement score, muscle endurance testing, 14-days raw acceleration data (expressed both as mean daily Euclidean norm minus 1 g (ENMO)), and quality of life questionnaires were recorded. Relationships between variables were investigated using Spearman correlation coefficient (ρ) and random forest (RF) regressions. Fifty-five patients (16 OM, 27 IMNM and 12 DM) were included. At M0, ENMO mainly correlated with manual muscle testing 8 (MMT8, ρ=0.44, p=0.001), creatinine level (ρ=0.43, p=0.001), HAQ (ρ=−0.53, p<0.001), and SF-36-physical functioning (ρ=0.40, p=0.002). According to RF regressions, most important features associated with ENMO Z-scores were HAQ, SF-36-physical functioning/energy/pain and disease duration. At M6, ENMO changes mainly correlated with changes in depression score (ρ=−0.57, p=0.001), HAQ (ρ=−0.51, p<0.001), and SF-36-energy/fatigue (ρ=0.51, p<0.001). According to the RF regression, most important features associated with ENMO changes were change in MMT8, HAQ, SF-36-physical functioning, physician global assessment, and depression score. Changes in ENMO were correlated with ACR/EULAR improvement score (ρ=0.57, p<0.01) and all patients, except one, achieving a major improvement at M6 had a significant ENMO change. Yet, variable changes in ENMO were observed in patients with minimal to moderate improvement at M6. PA levels were correlated with muscle strength and function, while PA change was also associated with emotional change. Only patients with major ACR/EULAR improvements at follow-up were associated with consistent changes in PA.