This study focuses on the role of social identity and prejudice as causal variables in public attitudes toward pre-exposure prophylaxis (PrEP) in a UK sample. In all, 222 participants participated in an experimental vignette study with a 2 × 2 × 2 design with between-participants factors of ethnicity, sexual orientation and gender, and completed measures of attitudes toward gay men, Black Africans, and PrEP. Kruskal–Wallis tests showed significant effects of gender on attitudes toward gay men and Black Africans; of ethnicity on attitudes toward gay men and PrEP; and of religion on attitudes toward gay men. A univariate analysis of variance (ANOVA) revealed more positive attitudes toward PrEP when it was presented as benefitting men (vs. women). Structural equation modeling showed that the relationship between the independent variables of gender, ethnicity, and religion and the dependent variable of attitudes toward PrEP was mediated by attitudes toward gay men and Black Africans. Individuals must perceive the principal beneficiaries of PrEP (i.e., gay men and Black Africans) positively to endorse PrEP for these groups, and sexism may reduce public acceptability of PrEP for women. Future research should use representative samples and alternative experimental manipulations, and include strength of social identification as an independent variable.