The aim of this study is to determine the effect of shared instructional leadership and social capital on school effectiveness according to teacher perceptions. The study was designed in a descriptive relational survey model. The sample of the study consists of 403 teachers working in schools in the Odunpazari and Tepebasi districts located in the city center of Eskisehir, Turkey in the 2021-2022 academic year. In the study, "Shared instructional leadership scale," "Social capital scale" and "School effectiveness scale" were used. In the study, it was found that teachers' perceptions of shared instructional leadership, social capital, and school effectiveness levels were high. It was determined that shared instructional leadership and social capital variables together had a high and significant relationship with school effectiveness. The relative importance of the predictor variables on school effectiveness are shared instructional leadership and social capital variables, and these variables were found to be significant predictors of school effectiveness. School principals can contribute to the strengthening of social capital and school effectiveness by exhibiting shared instructional leadership behaviors such as focusing on student learning, sharing authority and responsibilities, strong dialogue and cooperation.