While Uganda has achieved relatively high COVID-19 first dose vaccination rates, completion of the second dose remains low, particularly in Masindi district where only 44.0% of adults who were vaccinated with the first dose proceeded to receive the second dose. This shortfall threatens to undermine the achievement of herd immunity and the effectiveness of the nation's COVID-19 vaccination campaign. This study aimed to determine the rate of second dose vaccine uptake and assess factors associated with second dose uptake among adults in Masindi district. We conducted a community-based cross-sectional study using multi-stage random sampling approach to select 637 study respondents. Data were collected using a pre-tested interviewer-administered structured questionnaire and analyzed using SPSS v26. Modified Poisson regression was used to identify independent predictors of second dose uptake. Results were reported using adjusted prevalence ratios (APRs) and 95% confidence intervals (CIs), with statistical significance set at p