We study the impact of naturalization on the long-term social integration of immigrants into the host country society. Despite ongoing debates about citizenship policy, we lack reliable evidence that isolates the causal effect of naturalization from the non-random selection into naturalization. We exploit the quasi-random assignment of citizenship in Swiss municipalities that used referendums to decide on naturalization applications of immigrants. Comparing otherwise similar immigrants who narrowly won or narrowly lost their naturalization referendums, we find that receiving Swiss citizenship strongly improved long-term social integration. We also find that the integration returns to naturalization are much larger for more marginalized immigrant groups and somewhat larger when naturalization occurs earlier, rather than later in the residency period. Overall, our findings support the policy paradigm arguing that naturalization is a catalyst for improving the social integration of immigrants rather than merely the crown on the completed integration process.