The purpose of this research was to examine the moderating role of sibling relationship quality in the associations between shyness and indices of socioemotional adjustment in an early childhood education context. Participants were 79 children ages 4 to 6 (M = 4.74 years) who had at least one sibling. Parents completed ratings of child shyness, sibling relationship quality was assessed with parent and child self-report, and teachers and children completed assessments of child socioemotional functioning at preschool. Among the results, shyness and sibling relationship quality uniquely predicted adjustment at preschool. In addition, positive sibling relationships moderated relations between shyness and indices of preschool adjustment. For example, among children with less positive sibling relationships, shyness was more strongly associated with indices of internalizing problems. In contrast, these associations were attenuated among children with more positive sibling relationships. Results are discussed in terms of the protective role siblings may play in shy children's socioemotional adjustment at preschool. (Contains 2 figures, 2 tables and 4 notes.)