The need-to-belong theory stipulates that social exclusion (i.e., being rejected by peers) impairs the ability to self-regulate, and experimental studies with adults support this contention, at least on a short-term basis. Few studies have investigated whether social exclusion affects the development of self-regulation of children in a more enduring manner. By using data from a community sample of 762 children, we investigated reciprocal relations between social exclusion and self-regulation from age 4 to age 6. Social exclusion was reported by teachers, whereas self-regulation was reported by parents. Autoregressive latent cross-lagged analyses showed that social exclusion predicted impaired development of dispositional self-regulation and, reciprocally, that poor self-regulation predicted enhanced social exclusion. In other words, social exclusion undermines children's development of self-regulation, whereas poor self-regulation increases the likelihood of exclusion. Results illuminate the applied relevance of the need-to-belong theory.