Public health measures promoting compliance of COVID-19 vaccination requires understanding of knowledge, attitudes, and practices (KAP). This study explored the KAP and risk factors influencing COVID-19 vaccination, including changes in preventive practices before and after vaccination in a high-income country, Singapore. An online cross-sectional study among Singaporeans and permanent residents aged 21 years and older was conducted from July to August 2021. Univariate and multivariable logistic regressions using RStudio version 1.4.1106 was performed to assess associations between demographic factors, KAP, and vaccination status. P values < 0.05 were considered statistically significant. A total of 869 respondents completed the survey. Individuals with higher knowledge (adjusted odds ratio [aOR] = 2.00, P = 0.024), perceived efficacy (aOR = 1.19, P = 0.004), perceived safety (aOR = 1.20, P = 0.005), and willingness to uptake (aOR = 1.55, P < 0.001) scores were more likely to be vaccinated. There was a significant increase in the use of proper handwashing techniques among the vaccinated group before and after vaccinations. The governmental risk communication approaches have been useful in instilling high levels of vaccine knowledge. High levels of good attitudes about and knowledge of COVID-19 vaccination were associated with a high level of vaccination practices. Good perceived vaccine efficacy and confidence in government were also associated with positive vaccine uptake. This study paves the way for more targeted government measures to be implemented to improve vaccination rates of COVID-19 booster vaccines in a high-income country like Singapore.