Background: COVID-19 vaccination uptake is influenced by various sociodemographic and psychological factors. Previous studies have suggested that the relationship between personality traits and vaccination is context-dependent. However, this association has not been investigated in Japan, a unique sociocultural context characterized by historically low vaccine confidence but exceptionally high COVID-19 vaccine uptake. We examined the association between the Big Five personality traits and completion of the primary COVID-19 vaccination series, exploring potential gender-personality interactions. Methods: A web-based, cross-sectional survey was conducted among 7,210 Japanese adults (aged 20-85 years) in December 2021. Personality was assessed using the Japanese version of the Ten-Item Personality Inventory. Multivariable Poisson generalized estimating equations were used to calculate adjusted prevalence ratios (PRs) and assess gender-personality interactions. Results: Higher extraversion and lower openness were significantly associated with vaccination completion across genders. A significant gender-agreeableness interaction was observed: agreeableness (PR = 1.016, 95% CI: 1.008-1.024) was positively associated with vaccination completion among men but not women. A similar, though only marginally significant, interaction pattern was observed for neuroticism (PR = 1.011, 95% CI: 1.004-1.019 among men). Conclusions: Personality traits act as modest but meaningful, gender-specific psychological modifiers of vaccination behavior in Japan. Public health campaigns should transition from traditional "one-size-fits-all" messages to personality-tailored and gender-responsive communication strategies to optimize vaccine uptake and bolster psychological resilience in future pandemics.