In Indonesia, as in other countries, a large proportion of tax returns are filed at the last minute. In a population-wide randomised controlled trial (n = 11,157,069), we evaluated the impact of behavioural email prompts on the proportion of annual tax returns filed at least two weeks before the deadline; and overall filing rate. In two control conditions, taxpayers either received no email or an email used in prior years, emphasising regulatory information. The five treatments informed by behavioural science were (1) a simplified version of the existing email, emphasising early filing; (2) the simplified version with additional guidance on filing taxes; (3) the simplified version with a planning prompt and option to sign up for email reminders; (4) a version combining treatments 1, 2 and 3; and (5) an email appealing to national pride. Compared to the no-email control, all emails led to a statistically significant increase in early and overall filing rates.