There is limited understanding of how risk perceptions changed as the US population gained experience with COVID-19. The objectives were to examine risk perceptions and determine the factors associated with risk perceptions and how these changed over the first 18 mo of the pandemic. Seven cross-sectional online surveys were fielded between May 2020 and October 2021. The study included a population-weighted sample of 138,303 US adults drawn from a market research platform, with an average 68% cooperation rate. Respondents' risk perception of developing COVID in the next 30 days was assessed at each time point. We examined relationships between 30-day risk perceptions and various factors (including sociodemographic features, health, COVID-19 experience, political affiliation, and psychological variables). COVID risk perceptions were stable across the 2020 surveys and showed a significant decrease in the 2021 surveys. Several factors, including older age, worse health, high COVID worry, in-person employment type, higher income, Democratic political party affiliation (the relatively more liberal party in the United States), low tolerance of uncertainty, and high anxiety were strongly associated with higher 30-d risk perceptions in 2020. One notable change occurred in 2021, in that younger adults (aged 18-29 y) had significantly higher 30-d risk perceptions than older adults did (aged 65 y and older) after vaccination. Initial differences in perception by political party attenuated over time. Higher 30-d risk perceptions were significantly associated with engaging in preventive behaviors. Cross-sectional samples, risk perception item focused on incidence not severity. COVID risk perceptions decreased over time. Understanding the longitudinal pattern of risk perceptions and the factors associated with 30-d risk perceptions over time provides valuable insights to guide public health communication campaigns. The study assessed COVID-19 risk perceptions at 7 time points over 18 mo of the pandemic in large samples of US adults.Risk perceptions were fairly stable until the introduction of vaccines in early 2021, at which point they showed a marked reduction.Higher COVID-19 30-d risk perceptions were significantly associated with the preventive behaviors of masking, limiting social contact, avoiding restaurants, and not entertaining visitors at home.