ABSTRACT Citizenship offers enhanced opportunities for immigrants, but the complex naturalization process can be a barrier. We evaluate a preregistered, large‐scale randomized trial that tested the impact of a low‐cost letter that combined informational content with active encouragement from the United States Citizenship and Immigration Services (USCIS) to potentially eligible Lawful Permanent Residents (LPRs) to apply for naturalization. Within a nationwide population of 1,861,178 individuals in LPR status for 6 to 9 years, we find that the letter increased naturalization applications by 1 percentage point (12%). This increase was positive across all groups tested, though the effects were larger for families in higher income zip codes and those who did not submit the fee waiver application, suggesting that financial and time constraints, along with administrative burdens, play a role in naturalization submission rates.