The aim of this study was to develop and psychometrically validate a self-report instrument to assess (1) competences, (2) needs, and (3) satisfaction among youth transitioning from pediatric to adult rheumatology. The Transition-KompAZ was developed in several steps with conceptual and psychometric analyses. The instrument was administered to adolescents and young adults (AYA, 16-25 years) with inflammatory rheumatic diseases before (group 1) or after (group 2) transfer to adult rheumatology to test its psychometric properties. A two-factor higher-order confirmatory factor analysis (CFA) model was applied to test the hypothesized factor structure. Internal consistency was estimated using the approach of Raykov with the factor loadings and error variances estimated in the CFA. Spearman rank correlation coefficients were used to assess construct validity. The Transition-KompAZ includes the following modules: (1) competences in transition [knowledge; self-management], (2) needs [health care services, information], and (3) satisfaction [general; transitional care]. A total of 173 AYA (group 1: n=86, group 2: n=87) from 12 rheumatology sites completed the Transition-KompAZ. It showed good model fit (Comparative Fit Index >0.9, Tucker-Lewis Index >0.9; Weighted Root Mean Square Residual <0.9) with good internal consistency. The instrument demonstrated moderate to good construct validity, and good test-retest reliability. The Transition-KompAZ appears to be a reliable tool for assessing important dimensions of transition. It may support a structured and individualized transition and the evaluation of transition services. However, further studies are required to assess its predictive value in terms of transfer readiness and successful transition.