Background Combination prevention, which includes pre-exposure prophylaxis (PrEP), is essential for achieving the zero HIV infections target in the UK by 2030. It is important to assess attitudes towards PrEP in at-risk populations. This study focuses on the effect of discrimination and HIV conspiracy theorising on attitudes towards PrEP in gay men in the UK. In total, 244 White British gay men completed a survey that included demographic questions and measures of sexual health screening, hypervigilance, sexual orientation discrimination, quality of contact with healthcare professionals, belief in conspiracy theories and attitudes towards PrEP. Data were analysed using multiple linear regression and mediation analysis. Discrimination was positively correlated with HIV conspiracy beliefs and negatively correlated with PrEP acceptance. Mediation analyses demonstrated that the relationship between discrimination and attitudes towards PrEP was explained by HIV conspiracy theorising. Gay men who had attended a sexual health screening (vs never attended) reported higher belief in HIV conspiracy theories. A further mediation analysis showed that reported poor contact with a healthcare professional was associated with an increased belief in HIV conspiracy theories, which was associated with negative attitudes towards PrEP. Both perceived discrimination and poor contact with a healthcare professional were exacerbated by hypervigilance. HIV conspiracy theorising is an important variable in understanding attitudes towards PrEP among gay men. Its roots are in adverse social experiences (e.g. discrimination, poor contact with healthcare professionals) and its consequences may be the rejection of PrEP. HIV prevention and PrEP campaigns must focus on prejudice reduction and on challenging conspiracy beliefs.