The overarching goal of this 2-phase study was to investigate the contributions of relational reasoning to mathematical thinking and performance for 790 primary and middle-school Chinese students. Phase I of the study was undertaken to establish the reliability and validity of the Test of Relational Reasoning-Junior (TORRjr), a 32-item measure developed to assess children's and young adolescents' ability to reason analogically, anomalously, antinomously, and antithetically. The range of analyses from item difficulties to the overall structural model showed the TORRjr to be a psychometrically sound measure for study participants. There was also evidence of development in students' relational reasoning ability. In Phase II, the contributions of TORRjr performance to results of a novel measure of quantitative relations (QRTC[superscript 2]), a traditional measure of mathematical ability (MAT), and a fluid measure of mental rotation (MRT) were examined. Relational reasoning was significantly associated with performance on all 3 measures to a moderate or moderately strong level. Relational reasoning was also found to partially mediate the association between the QRTC[superscript 2] and MAT and fully mediate the relation between MRT and MAT. Multiple-group regression analyses revealed a stronger association between relational reasoning and mathematical performance for middle-school students than for primary students. Theoretical and practical implications of the outcomes from both phases are forwarded.