We examine how income inequality, measured by the Gini coefficient, moderates the relationship between Big Five personality traits and well-being, within the person-environment fit framework. Using data from all 50 U.S. states, we analyzed both subjective well-being (from Gallup surveys, including six sub-dimensions and an overall index) and objective well-being (via a Twitter happiness index), while controlling for education level, income, and demographic factors. Correlation and partial correlation analyses revealed that the Gini coefficient was significantly negatively correlated with multiple well-being indicators; however, this association weakened after controlling for covariates. Stepwise regression analysis identified significant moderating effects of the Gini coefficient. Specifically, higher income inequality weakened the positive impacts of extraversion, agreeableness, and conscientiousness on well-being (e.g., work environment and basic needs well-being), and intensified the negative effect of neuroticism on work environment well-being, indicating a poor person-environment fit. In contrast, openness demonstrated a positive fit with higher inequality, as its beneficial influence on Twitter-based well-being was amplified. These findings highlight that income inequality shapes how personality traits influence well-being, offering insights for targeted well-being interventions in diverse economic contexts.