This study’s purpose was to explore the influence of substance abuse on risky sexual behaviours among undergraduate students of main campuses in Nakuru County. Social Learning Theory by Albert Bandura and Person-Centred Theory by Carl Rogers guided the study. The study was carried out among 3rd, 4th, 5th and 6th year undergraduate students from main campuses in Nakuru County. The study adopted a correlational research design. The target population was 18570 undergraduate students. The accessible population was 8456 3rd, 4th, 5th and 6th year undergraduate students whose sample was 367. Simple random sampling technique was used to select the respondents. The study included a sample of 10 peer counsellors and 2 student counsellors from the Egerton University and Kabarak University main campuses who were selected through purposive sampling. Data were obtained using questionnaire for undergraduate students, focus group discussions guide for peer counsellors and interview schedule for student counsellors. Validation of research instruments was done through peer and expert review and also through pilot testing which was done in Mount Kenya University (MKU) - Nakuru Campus. Both descriptive and inferential statistics were used to analyse quantitative data using SPSS Version 25 whereas the qualitative data was analysed thematically. The analysed data was presented in tables, graphs, and narratives. The researcher considered ethics of confidentiality and informed consent. The findings indicated that majority of undergraduate students perceive that the use of alcohol, marijuana, cocaine, and injectable steroids could lead to risky sexual behaviours.