Foster youth have higher rates of psychotropic medication use and concurrent multiclass psychotropic polypharmacy compared with nonfoster youth. However, less is known about the extent of multiclass psychotropic polypharmacy after adjusting for patient factors associated with psychotropic medication use OBJECTIVES: To (a) compare psychotropic medication use and psychotherapy use by youth in foster care to those not in foster care in the Oklahoma Medicaid population across various sociodemographic and clinical factors, and (b) determine if patient-related characteristics are associated with high levels of concurrent multiclass psychotropic polypharmacy. This cross-sectional, retrospective analysis was conducted using paid prescription, outpatient, and inpatient Oklahoma Medicaid administrative claims from calendar year 2016. Foster youth and adolescents aged 20 years or younger were identified (n = 9,325) and compared with the general Oklahoma Medicaid population of the same age (n = 639,868). Descriptive statistics highlight baseline demographic and clinical differences between the 2 groups. Multivariable logistic regression was used to determine if covariates were associated with concurrent multiclass psychotropic polypharmacy. A subgroup analysis of foster youth taking at least 1 psychotropic medication was also performed to determine factors associated with the highest level of concurrent multiclass psychotropic polypharmacy. Foster care was associated with higher odds of concurrent multiclass psychotropic polypharmacy regardless of presence of psychotherapy. Among the subgroup of foster youth taking at least 1 psychotropic medication, attention deficit hyperactivity disorder medications were the most commonly prescribed medication class, followed by antidepressants and anxiolytics when use was not chronic. However, at the highest level of chronic multiclass psychotropic polypharmacy (4-5 chronic concurrent medications), antipsychotics rose to the top, and anxiolytics were the least likely to be prescribed. Overall, the foster care population had the highest proportion of individuals with concurrent multiclass psychotropic polypharmacy (9.2% vs. 1.9%, P < 0.0001). The highest level of chronic multiclass psychotropic polypharmacy was more likely to occur in males (OR = 1.66, 95% CI = 1.40-1.96) and patients living in group homes (OR = 4.13, 95% CI = 2.02-8.44) or foster homes (OR = 1.66, 95% CI = 1.25-2.19). Being overweight or obese was associated with an 83% higher odds of being at the highest level of concurrent multiclass psychotropic polypharmacy (95% CI = 1.27-2.64). Despite higher psychotherapy use, high rates of psychotropic medication use and concurrent multiclass psychotropic polypharmacy in foster youth remain a concern for policymakers. Patterns observed at different levels of concurrent multiclass psychotropic polypharmacy may be key to identifying youth who require additional monitoring. Future research exploring factors associated with higher levels of psychotropic concurrent multiclass psychotropic polypharmacy in foster youth can lead to actionable interventions and important policy changes. This project was funded through the CHIP Health Services Initiative. Keast, Tidmore, and Lambert report contractual employment for the Oklahoma Health Care Authority. Nesser is an employee of the Oklahoma Health Care Authority, and Shropshire is an employee of the Oklahoma Department of Human Services. Keast discloses unrelated research grant funding from AbbVie, Amgen, Otsuka, and Purdue Pharma. Tidmore discloses unrelated research grant funding from Amgen and Otsuka. The remaining authors have no relevant disclosures or conflicts of interest to declare. Posters based on this study were presented at AMCP Nexus 2017; October 16-19, 2017; Grapevine, TX, and at the AMCP Annual Meeting 2018; April 23-26, 2018; Boston, MA.