This study documents perceptions of the relative harmfulness of marijuana and alcohol to a person's health among adults in Oregon just before the first legal sales of marijuana for recreational use. We surveyed 1941 adults in Oregon in September 2015. Respondents were recruited using an address-based sampling (ABS) frame (n = 1314) and social media advertising (n = 627). Respondents completed paper surveys (ABS-mail, n = 388) or online surveys (ABS-online, n = 926; social media, n = 627). We used descriptive statistics and logistic regression models to examine perceptions of the relative harmfulness of marijuana and alcohol by sample characteristics, including substance use. About half of adults in Oregon (52.5%) considered alcohol to be more harmful to a person's health than marijuana. A substantial proportion considered the substances equally harmful (40.0%). Few considered marijuana to be more harmful than alcohol (7.5%). In general, respondents who were younger, male, and not Republican were more likely than others to consider alcohol more harmful than marijuana. Respondents who were older, female, and Republican were more likely to consider marijuana and alcohol equally harmful. Most individuals who reported using both marijuana and alcohol (67.7%) and approximately half of those who used neither substance (48.2%) considered alcohol to be more harmful than marijuana. Perceptions about the relative harmfulness of marijuana and alcohol may have implications for public health. As state lawmakers develop policies to regulate marijuana, it may be helpful to consider the ways in which those policies may also affect use of alcohol and co-use of alcohol and marijuana.