We tested the longitudinal measurement invariance of the Teacher Network of Relationships Inventory (TNRI), a teacher-report measure of teacher-student relationship quality (TSRQ), on a sample of 784 academically at-risk students across ages 6 to 15 years by comparing the model for each subsequent year with that of the previous year(s). The TNRI was constructed with 22 items to form 3 correlated factors: Warmth, Conflict, and Intimacy. Cronbach's alphas ranged from 0.87 to 0.96 across 9 years. Both metric and scalar measurement invariance held for 9 years, indicating that scores on the TNRI have similar meaning across these ages. The TNRI also demonstrated measurement invariance across gender and race/ethnicity. Findings support that the TNRI is an appropriate measure for investigating substantive issues related to developmental changes in TSRQ from early childhood through adolescence, including gender and ethnic/racial differences in TSRQ across these ages. Based on repeated-measures ANOVAs, each scale decreased across the 9 years, although the growth patterns for scales differed somewhat: Conflict had a linearly decreasing pattern, Warmth declined most notably as students make the transition to adolescence, whereas Intimacy scores dropped off noticeably at the transition from early to late childhood. Research limitations and implications for practice are discussed.