Objective: To evaluate the demographic, disease activity, disability, and health-related quality of life (HRQOL) differences between children with juvenile idiopathic arthritis (JIA) and their healthy peers, and between children with JIA with and without clinical temporomandibular joint (TMJ) involvement and its determinants. Methods: This study is based on a cross-sectional cohort of 3,343 children with JIA and 3,409 healthy peers, enrolled in the Pediatric Rheumatology International Trials Organisation HRQOL study or in the methotrexate trial. Potential determinants of TMJ involvement included demographic, disease activity, disability, and HRQOL measures selected through univariate and multivariable logistic regression. Results: Clinical TMJ involvement was observed in 387 of 3,343 children with JIA (11.6%). Children with TMJ involvement, compared to those without, more often had polyarticular disease course (95% versus 70%), higher Juvenile Arthritis Disease Activity Score (odds ratio [OR] 4.6), more disability, and lower HRQOL. Children with TMJ involvement experienced clearly more disability and lower HRQOL compared to their healthy peers. The multivariable analysis showed that cervical spine involvement (OR 4.6), disease duration >4.4 years (OR 2.8), and having more disability (Childhood Health Assessment Questionnaire Disability Index >0.625) (OR 1.6) were the most important determinants for TMJ involvement. Conclusion: Clinical TMJ involvement in JIA is associated with higher disease activity, higher disability, and impaired HRQOL. Our findings indicate the need for dedicated clinical and imaging evaluation of TMJ arthritis, especially in children with cervical spine involvement, polyarticular course, and longer disease duration.