The existence of a “socioeconomic achievement gap”—a disparity in academic achievement between students from high- and low-socioeconomic status (SES) backgrounds—is well-known in educational research. The SES achievement gap has been documented across a wide range of countries. What is unknown in most countries is whether the SES achievement gap might be changing over time. This study combines 30 international large-scale assessments over 51 years, representing 100 countries and about 5.8 million students. Results indicate that on average globally, SES achievement gaps have increased for all three available measures of family SES: parents’ education, parents’ occupation, and the number of books in the home. These trends are partially explained by expanding school access and declining fertility rates.