OBJECTIVE: Youth with juvenile idiopathic arthritis (JIA) experience elevated rates of internalizing symptoms, although more research is required to understand this phenomenon. Perfectionism, a multidimensional personality trait that involves dimensions such as striving for flawlessness (self-oriented perfectionism) and feeling that others demand perfection (socially-prescribed perfectionism), is a well-known risk factor for internalizing symptoms that has received minimal attention in pediatric populations. Preregistered hypotheses explored the relationships between youth and parent perfectionism and symptoms of depression and anxiety in youth with JIA, as mediated by (a) youth/parent negative self-evaluations and (b) youth self-concealment. METHODS: One hundred fifty-six dyads comprised of youth (13-18 years) with JIA and a caregiver completed online questionnaires about trait perfectionism, negative self-evaluations (i.e., pain catastrophizing and fear of pain), self-concealment, and internalizing symptoms. RESULTS: Positive relationships were observed between parent/youth self-oriented perfectionism and negative self-evaluations, youth self-oriented perfectionism and internalizing symptoms, and youth negative self-evaluations and internalizing symptoms. Parent self-oriented perfectionism was negatively related to youth depression symptoms. Indirect effects were observed for youth self-oriented perfectionism predicting anxiety and depression symptoms through pain catastrophizing (a1b1 = 0.13 and 0.12, 95% CI [.03, .24 and .03, .22], respectively). Exploratory mediations suggested youth socially-prescribed perfectionism might predict internalizing symptoms directly and indirectly through self-concealment. CONCLUSION: Youth and parent perfectionism are implicated in the internalizing symptoms of youth with JIA and may manifest through youth negative self-evaluations (e.g., catastrophic thoughts) and self-concealment. While future research is needed, screening for perfectionistic tendencies in this population may help guide assessment, prevention, and treatment efforts.