This study investigates the relationship between obesity and academic achievement using cross-sectional data from 52 countries for the period 2012–2022. Academic success is measured by PISA scores in reading, science, and mathematics, while the obesity rate serves as the main explanatory variable. Thinness, per capita income, and the education expenditure ratio are included as control variables. Using the Ordinary Least Squares (OLS) method, the analysis reveals that obesity has a negative effect on students’ performance across all three domains. However, the negative impact of thinness on academic achievement is even stronger than that of obesity, suggesting that undernutrition may hinder cognitive development more severely than excess body weight. Moreover, per capita income is positively and significantly associated with academic achievement, whereas the share of education expenditure in GDP shows no statistically significant effect. These results highlight the strong association between students’ physical health and their academic outcomes, emphasizing the need for integrated education and public health policies to students’ overall learning outcomes.