Although sluggish cognitive tempo (SCT) is associated with poorer peer functioning, no study has examined SCT in relation to student-teacher relationship quality. The current study examined whether SCT, as rated by both teachers and children, was uniquely associated with poorer student-teacher relationship quality above and beyond child demographics and other mental health symptoms (i.e., attention-deficit/hyperactivity disorder [ADHD], oppositional defiant disorder/conduct disorder [ODD/CD], anxiety/depression). Gender was examined as a possible moderator of the association between SCT and student-teacher relationship quality. Participants were 176 children in 1st-6th grades and their teachers. Teachers rated children's SCT and other mental health symptoms in the fall semester (T1) and the student-teacher relationship (conflict and closeness) 6 months later (T2). Children provided self-ratings of SCT at T2. Above and beyond age, gender, and other mental health symptoms, teacher-rated SCT at T1 was associated with greater student-teacher conflict at T2. This association was qualified by a SCT × Gender interaction, with SCT associated with greater conflict for girls but not boys. Further, child-rated SCT was also associated with greater teacher-rated conflict, above and beyond covariates. In addition, teacher-rated SCT at T1 was the only mental health dimension to be significantly associated with less student-teacher closeness at T2. Findings extend the social difficulties associated with SCT to the student-teacher relationship, an important relationship associated with children's academic and socioemotional outcomes.