We examined the influence of language and socioeconomic factors on academic achievement in the Basque Autonomous Community (BAC): a multilingual society with a recent history of language revitalisation. We analysed scores obtained by Grade 4 and Grade 10 students on a diagnostic mathematics test administered in either Basque or Spanish in the BAC. We found that L1 Basque users who were of high socioeconomic status (SES) were the top performers across language groups. A larger percentage of high-SES L1 Spanish users earned top scores when tested in their home language than when tested in Basque. In contrast, low-SES L1 users of Spanish performed better when tested in Basque. These findings corroborate and expand upon findings from previous research conducted in the BAC, which shows that SES and language shape academic achievement in contexts of successful language revitalisation efforts.