This study provides the first ever longitudinal, globally comparative analysis of the level of socioeconomic segregation and achievement inequality. It is based on a unique pooled dataset containing harmonized indicators for student socioeconomic status (SES) derived from ten international large-scale assessments in education, covering approximately 9 million students in 300,000 schools in 143 countries. I find that levels of socioeconomic school segregation vary greatly across countries and regions and are particularly high in low- and middle-income countries in the global South. School segregation is negatively associated with economic development, and positively associated with achievement inequality, economic inequality, and the share of private school enrolment. Based on these findings, I argue that many countries in the global South are stuck in a negative equilibrium of highly segregated education systems and high levels of educational and social inequality.