Background: Although previous research indicates an association between school connectedness and adolescents' sleep quality, its causal direction has not been determined. This study used a 2-wave cross-lagged panel analysis to explore the likely causal direction between these 2 constructs. Methods: Participants were 888 Chinese adolescents (43.80% boys; M[subscript age] = 15.55) who provided self-report data on school connectedness and sleep quality as well as demographic variables at the beginning and the end of a school year. Results: After controlling for sex and age, we found that sleep problems at the beginning of the school year were a significant and negative predictor of school connectedness at the end of the school year (b[subscript 2] = -0.26, SE = 0.13, ß[subscript 2] = -0.10, p < 0.05), but school connectedness at the beginning of the school year did not predict sleep problems at the end of the school year (b[subscript 1] = 0.05, SE = 0.03, ß[subscript 1] = 0.09, p > 0.05). Separate analyses by sex showed that the above pattern of results was mainly driven by the boys. Conclusions: Our findings demonstrated that sleep problems could be a risk factor for adolescent boys' school connectedness.