This research investigated whether parent-teacher relationship quality mediated the relation between parents' motivational beliefs and children's adaptive functioning and externalizing behaviors. The sample consisted of kindergarten through third-grade children with behavioral concerns (N = 206). Parents reported on their motivational beliefs (i.e., role construction and efficacy), and teachers reported on the quality of their relationships with parents and children's adaptive functioning (i.e., social and adaptive skills) and externalizing behaviors. Results indicated that parents' motivational beliefs were related significantly and positively to children's adaptive functioning and negatively to children's externalizing behaviors. Parents' motivational beliefs were also significantly associated with enhanced parent-teacher relationship quality. There was a significant medium-sized indirect effect of parents' motivational beliefs on children's adaptive functioning through parent-teacher relationship quality (k[superscript 2] = 0.12) and a small indirect effect of parents' motivational beliefs on children's externalizing behaviors (k[superscript 2] = 0.05). This research suggests that parent-teacher relationship quality may be one mechanism by which the benefits of parents' motivational beliefs are transmitted to children. (Contains 1 table and 3 figures.)