OBJECTIVES:  To evaluate the prevalence and determinants of long-term multiclass psychotropic polypharmacy (PP) among children and adolescents with mental disorders. METHODS:  A retrospective cross-sectional study was conducted using the 2013-2015 administrative claims data from Texas Children’s Health Plan. PP was defined as the receipt of ≥ 2 psychotropic medications from different drug classes concurrently for 60 days or more. Based on the number of prescribers involved in the treatment, the patients were categorized into two groups: a) single prescriber (SP) and b) multiple prescribers (MP). Logistic regression models and the Farilie decomposition method (extension of Blinder-Oaxaca [BO] decomposition) were conducted to assess the relative importance of determinants of PP based on the Andersen Behavioral Model. RESULTS:  A total of 24,147 children and adolescents with a diagnosis of mental disorder and prescription of psychotropic medication were identified. The prevalence of PP was 20.09%. Logistic regression analyses revealed that patients with specialist involvement (enabling factor) had 5.3 and 3.6 times higher likelihood of receiving PP in the SP (OR=5.32; 95% CI 4.62-6.14) and MP (OR=3.57; 95% CI 3.20-3.99) groups, respectively. Other significant factors associated with PP were patient race (predisposing factor) and diagnosis of bipolar disorders and depression, as well as the number of mental disorders diagnosed (need factor) and number of prescribers involved in treatment (MP group only). The Farilie decomposition analysis estimated that the observed need factors explained only 25% of the difference in the receipt of PP between patients seen by PCPs and specialists within both SP and MP groups. CONCLUSIONS:  The most prominent enabling factor associated with PP was involvement of a specialist in the treatment of mental disorders. Only one-fourth of the difference between PCPs and specialists’ prescribing of PP was explained by observable need factors, underscoring the importance of evaluating different prescribing practices by PCPs and specialists.