This study examined the causal relationships among teacher-student relationships, depression, and academic achievement in adolescents during the transition from middle to high school. To achieve this, data from the Daegu Education Longitudinal Study were utilized, specifically from the 5th (2021) to the 7th (2023) waves of the 2017 cohort of 4th-grade elementary school students. After excluding cases with missing values or discrepancies in middle school codes, a total of 1,451 students' data were analyzed using an autoregressive cross-lagged model. The findings of the study were as follows: First, the teacher-student relationship, depression, and academic achievement at the previous time point positively influenced these same variables at the subsequent time point, with no significant gender differences. Second, regarding the cross-lagged effects, teacher-student relationships and depression negatively influenced each other, while academic achievement positively influenced teacher-student relationships. When examining gender differences, for male students, teacher-student relationships negatively impacted both depression and academic achievement. In contrast, for female students, depression negatively influenced teacher-student relationships, while academic achievement positively influenced teacher-student relationships. Based on these results, it is hoped that they will serve as foundational data for developing various programs to enhance teacher-student relationships and for establishing effective processes to detect and manage depression at an early stage.