Research indicates social anxiety is associated with lower friendship quality, but little is known about the underlying mechanisms. This 2-month longitudinal study examined social withdrawal as a mediator of the social anxiety–friendship quality link in a sample of 214 adolescents (Mage = 13.1 years, SD = .73) that included an oversampling of adolescents recently relocated to the community (n = 155). Findings provided preliminary support for the hypothesized models, in which social anxiety is associated with social withdrawal, which in turn is related to lower companionship and intimacy in adolescents’ friendships. Analyses testing whether relocation to a new community intensifies these associations indicated additive, but not multiplicative, effects of social anxiety and relocation on friendship companionship and intimacy. Implications include the importance of increasing socially anxious youths’ social engagement and skills with friends as well as with less familiar peers.