Research across various contexts shows that granting immigrants host-country citizenship is a powerful policy tool for promoting their economic and political integration and fostering social cohesion. However, persistently low naturalization rates limit the potential benefits of citizenship, leaving integration opportunities unfulfilled and constraining both immigrant and host communities. To address this challenge, we partnered with the City of Zurich, Switzerland, co-designing and implementing two large-scale outreach campaigns to encourage citizenship applications. In the first study, we randomly assigned over 10,000 newly eligible immigrants into treatment and control groups, delivering an informational letter encouraging citizenship application. Results showed a 41% (est: 0.025, 95% CI: 0.014-0.036) higher application rate in the treatment group. Survey responses revealed increased awareness of eligibility requirements and positive attitudes towards citizenship. Building on these findings, a second study targeted specific barriers—perceived unwelcomeness, process complexity, and lack of knowledge about eligibility requirements—via three targeted interventions. Treatments addressing lack of knowledge about requirements and unwelcomeness led to significant application rate increases of 69% (est: 0.043, 95% CI: 0.019-0.066) and 63% (est: 0.038, 95% CI: 0.016-0.061), respectively. In contrast, information letters aimed at reducing the perceived complexity of the application process had a more modest effect of 26% (est: 0.016, 95% CI: -0.006-0.038). Our research identifies critical obstacles in the naturalization process and demonstrates the effectiveness of targeted outreach in overcoming these barriers. These findings contribute to broader discussions on citizenship policy and immigrant integration, providing actionable strategies for policymakers to facilitate naturalization and strengthen societal cohesion.