It is typically assumed that language and national identity are strongly related. However, it is unclear whether the acquisition of a “national” language causes nationalist attitudes. Empirically, it is very difficult to test whether learning a native language causes nationalistic preferences since the correlation between being proficient in the language and nationalistic attitudes could easily be spurious. Hence, in order to answer the question on the causal impact of language acquisition on nationalist attitudes a quasi-experimental design is needed. We do find such a natural experiment in the case of the Basque Country where due to an educational reform the Basque language expanded very quickly after democratization. As a result, for a specific cohort it mattered in important ways in which year they were exactly born for whether they would learn Basque or not. Hence, year of birth is used as an instrument for Basque knowledge to study its causal effect on nationalist attitudes. Our study shows that Basque language acquisition does have a substantial causal impact on nationalist attitudes. Additionally, the paper test different causal mechanisms on the effect of language on national identity and provides evidence that the results can be generalized to other cases of minority nationalism. The study contributes to literatures on nation building, minority nationalism, and the assimilation of immigrants.