We tested a mediation model of the influence of teacher-student relationship quality on student group performance in a higher-education context where a group-oriented learning approach is implemented. Specifically, we posit that the relationship between teacher-student relationship quality at the group level and group performance is mediated by positive affective group well-being and intragroup communication quality. Data were collected from 68 groups of students at four time points. The hypotheses involved in the proposed mediated model were tested by means of multiple regression. We controlled for group size and students' initial familiarity with the other group members. All the analyses were conducted at the group level by means of the PROCESS macro for SPSS. The results obtained showed that teacher-student relationship quality has an indirect positive influence on group performance, mediated by positive affective group well-being and within-group communication quality. These findings help to understand how and why teacher-student relationship quality is related to student group performance.