The availability of an effective vaccine against COVID-19 virus marked a crucial moment in the fight against its pandemic spread. Although distribution of the vaccine began in December 2020, high acceptance rates and repeated administrations are needed to achieve widespread immunization, but hesitation toward the vaccine persists to this day. To identify psychological variables and other factors associated with vaccine hesitancy, we conducted a study from August 2021 to October 2022. An Internet-based survey gathered data from 137 Italian adults, exploring attitudes, sociodemographic characteristics, psychological variables, and immunization behavior. The results analysis showed that gender (69.2% of vaccine-adverse people were males), education (years of education was negatively correlated with vaccine hesitancy), and religion (not declaring oneself religious or atheist was more likely to be associated with hesitancy toward the vaccine) were the variables influencing attitudes toward the vaccine. Other psychological variables differentiated people with opposite attitudes toward the vaccine: high scores on the Conspiracy Mentality Questionnaire (CMQ) and Core Belief Inventory (CBI) were positively correlated with vaccine hesitancy, indicating that individuals with more pronounced core belief violation, due to the pandemic, tend to express higher levels of vaccine hesitancy. Finally, a linear regression analysis confirmed the role of participants’ conspiracy mentality as a valid predictor for vaccine hesitancy.