Girls typically perform better in reading than in mathematics or science, whereas boys typically perform better in mathematics or science than in reading. We assessed these sex differences in intraindividual academic strengths using data from 1.6 million adolescents across 82 countries and regions for three waves (2012, 2015, and 2018) of the Programme for International Student Assessment (PISA) among high (95th percentile), average (> 5th to < 95th percentile), and low (5th percentile) achievers. Girls’ intraindividual strength in reading and boys’ strength in mathematics or science were stable across countries, waves, and achievement levels. For countries in which boys had larger advantages in mathematics or science as an intraindividual strength, girls had an even larger advantage in reading. In line with a gender equality paradox, the magnitude of the sex differences in reading and science as intraindividual strengths increased with increases in national gender equality at each PISA achievement level. Interaction models suggest that the paradox arises because, as national gender equality increases, the sex with an overall advantage improves on their intraindividual strength, while the sex with an overall disadvantage shows a decline. The results have implications for understanding sex disparities in science, technology, engineering, and mathematics (STEM) fields.