Risky sexual behaviour is a common habit among young people in the world, and its relation with personality has not been explored exhaustively, especially in Kenya. The purpose of the study was to establish the relationship between personality sub-types and involvement in risky sexual behaviour among secondary school students in Nyakach Sub-county, Kisumu County, Kenya. The study was guided by the following research objectives; to establish the relationship between introversive personality subtype and involvement in risky sexual behaviour; to find out the relationship between extroversive personality subtype and involvement in risky sexual behaviour; to determine the relationship between the neurotic personality subtype and involvement in risky sexual behaviour and to investigate the relationship between the Psychoticism and involvement in risky sexual behavior. The study was informed by two theories that is, the Eysenck’s Theory of personality and the Theory of Planned Behavior. The research design used in this study was the Correlational research design. The sample size of this study was 46 schools representing 88% of the target population. As for the students, 390 students were sampled from total target population 15,678. This study used multistage random sampling to select the schools and students. Quantitative data was collected using Eysenck’s Personality Questionnaire (EPQ) and Risky Sexual Behaviour Questionnaires. Quantitative data was analyzed using descriptive statistics (frequency, percentages and mean) as well as inferential statistics (Pearson Correlation and multiple regressions). The major findings of the study showed relationship between personality sub-types and involvement in risky sexual behaviour (for introversion r=-0.439, extroversion r=0.786, neuroticism r=-0.410 and Psychoticism r=0.025). Multiple regression at R2=0.679 showing that personality subtypes are significant predicators of involvement in risky sexual behaviour at 67.9%. Reliability of questionnaires was obtained through internal consistency with cronbach’s constant at 0.632.The study recommended that the Ministry of Education should develop working guidelines to educate and mould the students in a way that they reduce their risk of engaging in risky sexual behaviour. Categorizing and profiling the students into their most dominant personality subtypes is one way that can be used to offer strategic counseling so as to handle various students depending on their personality subtypes. This will help reduce the tendency of students to involve in risky sexual behaviour. Stringent policy and laws on alcohol distribution should be developed and determined to ensure students, especially those who are considered to be underage, do not access alcohol since their risk of engaging in risky sexual behaviour increases with the consumption of alcohol and other drugs. The Ministry of Education should ensure that sex education as contained in the syllabus at various levels should be detailed and emphatic on the use of condoms so as to be able to debug the myths and negative attitudes that students have towards the use of condoms.