Background/Objectives: Vaccination significantly reduces mortality from COVID-19; however, uptake has declined, with most vaccine hesitancy observed among young adults. Reasons behind low COVID-19 vaccine uptake in this population are poorly understood. The aim of this study was to investigate the role of conspiracy theory mentality and vaccination beliefs as potential predictors of COVID-19 vaccination adherence behaviours (i.e. vaccine hesitancy and uptake). Methods: A cross-sectional design was adopted using an online survey, where validated questionnaires adapted for COVID-19 were distributed to one university cohort and on social media platforms targeting young adults in the UK (students aged 18–25-years-old). Quantitative measures included beliefs about vaccinations (Beliefs about Medicines Questionnaire [BMQ] – BMQ-Specific adapted for COVID-19 vaccination and BMQ-General adapted for vaccinations in general), conspiracy theory mentality (Vaccine Conspiracy Beliefs Scale - COVID-19) and vaccine hesitancy (Oxford Vaccine Hesitancy Scale). Vaccine uptake was measured by capturing the number of self-reported doses of COVID-19 vaccination received. Demographic characteristics were also collected and linear regression analysis conducted to identify determinants of vaccination behaviours. Results: One hundred and sixty-three valid responses were analyzed. All adapted scales showed acceptable internal consistency (Cronbach’s alpha values >0.64). COVID-19 vaccination beliefs (BMQ-Necessity-Concerns Differential), age and conspiracy mentality were significantly associated with vaccine hesitancy (F=76.6; Variance = 71.2%, adjusted R2=0.703). Only COVID-19 vaccine beliefs (Necessity-Concerns Differential) was predictive of vaccine uptake (F=14.866, Variance = 22.9%, adjusted R2=0.214). Increasing age was also associated with more negative vaccination beliefs (BMQ-Concerns - Beta=0.707, t=6.824, p<0.001: BMQ-Necessity- Beta =-0.882, t=-9.558, p<0.001) and vaccine hesitancy (Beta=1.976, t=2.481, p<0.05), but not vaccine uptake. Conclusions: These findings indicate that effective strategies to decrease vaccine hesitancy and promote vaccine uptake among young adults in the UK should aim to modify the underlying psychological factors that drive misconceptions about COVID-19 vaccination and conspiracy beliefs.