ObjectiveKnowledge about transdiagnostic factors associated with global symptom severity among patients diagnosed with various mental disorders remains limited. This study examined the cross-sectional associations between transdiagnostic psychological processes (emotional regulation, emotional intelligence, sleep quality, perceived stress, fear of the coronavirus, repetitive negative thinking), amount of physical activity, and sedentary behavior with global symptom severity, while controlling for sociodemographic data (age, gender, employment status, relationship status, educational level).MethodsData from 401 outpatients, aged 42.08 years on average (SD = 13.26; 71.3% female), diagnosed with depressive disorders, non-organic primary insomnia, agoraphobia, panic disorder, and/or posttraumatic stress disorder were analyzed. Data was collected from 10 different study sites between March 2021 and May 2022 for cross-sectional analysis. The influence of predictors of global symptom severity was determined using multiple regression. Global symptom severity was measured using the Global Severity Index, derived from the Brief Symptom Inventory-18. Predictors were measured using validated scales and physical activity was additionally assessed via accelerometer-based sensors.ResultsThe proposed model explained 40.3% of variance on the Global Severity Index. Higher levels of repetitive negative thinking, fear of coronavirus, perceived stress, worse sleep quality as well as greater difficulties with emotional regulation and emotional intelligence were significantly associated with higher global symptom severity in our clinical outpatient sample; however, no associations were found for physical activity, and sedentary behavior.ConclusionTransdiagnostic psychological processes were significantly associated with global symptom severity. Additional prospective longitudinal studies with transdiagnostic samples are necessary to explore possible causal relationships.Key Practitioner MessageRepetitive negative thinking, perceived stress, sleep quality, and emotional regulation can be considered distinct and individual transdiagnostic psychological processes associated with global symptom severity among outpatients diagnosed with various mental disorders.Treatment approaches aiming at improving transdiagnostic predictors of various mental disorders might be an efficacious target in reducing global symptom severity.Among outpatients (75% already receiving psychological or pharmacological treatment) diagnosed with depressive disorders, non-organic primary insomnia, agoraphobia, panic disorder, and/or PTSD, sociodemographic factors appear not to be predictive for global symptom severity.