Abstract Adult survivors of pediatric central nervous system (CNS) tumors are at the highest risk for morbidity and late mortality among all childhood cancers, partially due to chronic health conditions intersected with environmental and lifestyle factors. We aimed to study the prevalence of and risk factors for malnourishment (underweight, overweight, obese) amongst adult survivors of pediatric CNS tumors. We conducted a cross-sectional study assessing young adults (18-39 years) previously treated for pediatric CNS tumors who were followed in our survivorship clinic during 2016-2021. Sociodemographic, clinical, and treatment information was extracted from medical records with BMI status, per CDC guidelines, obtained from the most recent clinic visit. Data was assessed using one-way ANOVA, fishers exact test, and multivariable logistical regression. We identified 198 survivors (53% female, 84.3% white) categorized as underweight (4.0%), normal weight (40.9%), overweight (26.8%), obese (20.2%), and severely obese (8.1%). Patients were median age 8 years [4-12] at diagnosis and 24 years [20-28] at follow-up. Most common primary diagnosis and tumor location was low grade glioma (50.5%) and posterior fossa (39.4%) with 90.4% receiving surgery, 51.5% chemotherapy, and 52.5% radiotherapy. Male sex (OR 2.376, p = 0.006), hypothalamic/optic pathway tumor location (OR 2.362, P = 0.031), and radiation therapy (OR 2.844, p = 0.005) were associated with increased overweight/obesity risk, while chemotherapy (OR 0.363, p = 0.006) was associated with decreased risk. Male sex (OR 0.511, p = 0.007) and white ‘race’ (OR 0.166, p = 0.022) were associated with decreased underweight risk. The majority of young adult survivors seen in our clinic were not healthy weight by BMI, and the distribution of BMI groups appears comparable to contemporary studies of the general young adult U.S. population. The prevalence of malnourishment amongst PNO survivors warrants universal screening efforts with more accurate determinants of body composition than BMI, risk stratification, as well as targeted interventions.