There is emerging evidence that young people with first-episode psychosis are at greater risk of sexually-transmitted infections (STI) than their peers. Theoretical constructs central to behavioural change theories, broadly defined as sexual health-related knowledge, attitudes, and beliefs, have guided most sexual risk-reduction interventions in other at-risk populations. The role of these constructs in the sexual risk behaviour of young people with early psychosis remains unknown. A convenience sample of 67 young people with first-episode psychosis and 48 healthy controls matched on a number of sociodemographic characteristics was recruited. Participants completed a survey assessing their sexual behaviour and sexual health-related knowledge, attitudes, and beliefs. Group differences and the role of these constructs in the condom-use behaviour of these young people were examined. Although some differences emerged, group similarities were prominent. Inconsistent condom use was predicted by clinical status, unemployment, and the absence of peer support for condom use. These results support previous findings that young people with psychosis have greater needs for STI prevention due to increased rates of unprotected sex. Risk-reduction interventions that target peer influence are important. Inquiry into a broader range of psychosocial factors could further our understanding of STI infection risk in early psychosis.