Background: While existing evidence strongly suggests that immigrant students underperform relative to their native counterparts on measures of mathematics, science, and reading, country-level analyses assessing the homogeneity of the immigrant achievement gap across different factors have not been systematically conducted. Beyond finding a statistically significant average achievement gap, existing findings show considerable variation. The goal of this quantitative synthesis was to analyze effect sizes which compared immigrants to natives on international mathematics, reading, and science examinations. Methods: We used data from the Trends in International Mathematics and Science Study (TIMSS), the Programme for International Student Assessment (PISA), and the Progress in International Reading Literacy Study (PIRLS). We investigated whether the achievement gap is larger in some content areas than others (among mathematics, science, and reading), across the different types of tests (PISA, TIMSS, PIRLS), across academic grades and age, and whether it has changed across time. Standardized mean differences between immigrant and native students were obtained using data from 2000 to 2009 for current Organisation for Economic Co-operation and Development (OECD) countries. Results: Statistically significant weighted mean effect sizes favored native test takers in mathematics (d-bar*[subscript math]=0.38), reading (d-bar*[subscript reading]=0.38, and science (d-bar*[subscript science]=0.43). Effects of moderators differed across content areas. Conclusions: Our analyses have the potential to contribute to the literature about how variation in the immigrant achievement gap relates to different national-level factors.