Whether schooling systems with greater autonomy offset or reinforce educational inequalities remains debateable. Yet school autonomy is often advocated by different actors including donors and international organisations in many countries. This study examines the association between school autonomy and the inequality in maths achievement of 15-year-olds by socioeconomic status (SES), and whether this correlation differs by countries' level of development. We construct a country-level panel using six waves of the Programme for International Student Assessment (PISA) from 69 countries in 322 country waves. We first estimate the gradient of SES and maths achievement as a measure of achievement inequality at the country level and take the country mean of the relevant covariates including school autonomy. Results from weighted mixed-effects models suggest that the association between all autonomy variables and inequality in achievement is not significant. However, academic autonomy is linked to increasing achievement inequality in high- and upper-middle-income countries compared with low- and lower-middle-income countries. But the coefficients for budget and personnel autonomy do not differ by the development level. The results remain similar in several specification tests including country and year fixed effects, which leads us to suggest not to simplistically apply autonomy reforms regardless of context.