Students' sense of belonging at school has declined across the world in recent decades, and more so in Sweden than in almost any other high-income country. However, we do not know the characteristics or causes of these worldwide trends. Using data on Swedish students aged 15–16 years from the Programme for International Student Assessment (PISA) between 2000 and 2018, we show that the decline in school belonging in Sweden was driven by a disproportionately large decline at the bottom part of the distribution, and was greatest for foreign-born students, students from disadvantaged social backgrounds, and for low-achieving students. The decline cannot be accounted for by changes in student demographics or observable characteristics related to the school environment. The decline did, however, coincide with a major education reform, characterized by an increased use of summative evaluation, and an overall stronger performance-orientation.