While the HIV/AIDS epidemic has informed the responses to the COVID-19 pandemic, there is a lack of understanding of the empirical links between stigmas associated with COVID-19 and HIV/AIDS. Surveying 247 adults in the United States online, this study aims to examine the relationships between COVID-19 and HIV/AIDS stigmas in order to understand how they are similar and different from each other, taking into consideration the context relevant to the COVID-19 pandemic. Four scales of stigmas conveying different attitudes (i.e., rejection, despise, accusation, and caution) were used to examine the relationships between the stigmas of these two infectious diseases. Findings based on the mean comparisons from paired tests showed the mean difference in scores between respondents who supported the exclusion of COVID-19-infected individuals versus HIV-infected individuals was statistically significant. In addition, the mean difference in scores between respondents who were wary of their need to protect their rights around COVID-19-infected persons versus HIV-infected persons was statistically significant. Controlling for the respondents' sociodemographics and factors that were relevant to the COVID-19 pandemic, multiple regression analyses showed that all four types of COVID-19 stigmas were positively associated with their corresponding types of HIV/AIDS stigmas. The preliminary findings between COVID-19 and HIV/AIDS stigmas suggested that HIV/AIDS stigma mitigation strategies may present a fruitful approach to inform stigma mitigation of COVID-19.