Young adults experience high coronavirus disease 2019 (COVID-19) incidence yet have the lowest vaccination and booster rates among adults. Understanding the factors influencing their intentions regarding boosters is essential for crafting effective public health strategies. We examined the psychosocial factors (attitudes, norms, perceived behavioral control) associated with their intentions to receive a COVID-19 booster. This cross-sectional study included 292 young adults aged 18–25 residing in Philadelphia who completed an online survey from September 2021 and February 2022 (mean age 21.98, standard deviation 2.25; 51% racial/ethnic minorities). The survey included measures of attitudes, norms, and perceived behavioral control related to COVID-19 vaccination. We employed structural equation modeling analysis to examine the intention of young adults to receive the COVID-19 booster and their vaccine-related attitudes, norms, and perceived behavioral control. Covariates included race/ethnicity and gender. Subjective norms were significantly associated with the intention to receive a COVID-19 booster (standardized β̂ = 0.685, p = .018). Attitudes and perceived behavioral control showed no significant association with intention. Subgroup analyses based on race/ethnicity revealed that attitudes (standardized β̂ = 0.488, p = .004) and subjective norms (standardized β̂ = 0.451, p = .050) were predictors among young adults from racial and ethnic minority backgrounds, while only subjective norms (standardized β̂ = 1.104, p = .002) were significant for non-Hispanic White young adults. Public health efforts should prioritize engaging healthcare providers and peer groups in order to influence subjective norms and promote collective responsibility and acceptance for vaccination. Tailored interventions and diverse communication strategies targeting specific subgroups of young adults may be useful to ensure comprehensive and effective vaccination initiatives.