Integrating green practices into supply chain management is an important issue for companies to combine both environmental responsibility and the goal of increasing profits. In this paper, we present a meta-analysis of the relationship between green supply chain management (GSCM) practices and firm performance. By using robust variance estimation, we can appropriately analyze dependent effect sizes that are common in the studies to be included in this meta-analysis. In this way, more information is extracted from the primary studies and the effects of moderator variables can be examined simultaneously. Based on 408 correlation coefficients (N = 30,568) from 134 studies, we find a significant mean positive relationship between GSCM practices and firm performance in general (r = 0.442), and in particular, for market-based, managerial, and accounting-based performance. However, the correlations vary considerably across studies. To explain this heterogeneity, we analyze the influence of different GSCM practices and organizational characteristics.