Education plays an important role in curbing the spread of HIV and AIDS among the youth. However, there is little known how teachers' motivation in teaching HIV/AIDS education affects students' knowledge and attitudes towards sexual behaviour. The purpose of the study was to determine the relationship between teachers' level of motivation in teaching HIV/AIDS education and students' knowledge and attitude towards sexual behaviour in secondary schools in the Coast Region of Kenya. The study used descriptive survey research design. Proportional, purposive and random sampling methods were used to choose the participants. The samples comprised 421 respondents of which 33 were teachers and 388 students in 13 secondary schools. Questionnaires, document analysis and interview schedule were used to collect the data. Reliability coefficients of the instruments were 0.8 for both teachers and students. Both descriptive and inferential statistics were used in data analysis. The descriptive statistics that were used were frequencies and percentage. Chi-squire statistics was used to test the hypothesis in the study. The hypothesis testing was done at a = 0.05 level of significance. The data collected in this study was analysed using Statistical Package for Social Sciences (SPSS) computer programme version 20.0. The findings showed that there is a relationship between teachers' level of motivation in teaching of HIV/AIDS education and students' knowledge and attitudes towards sexual behaviour. It further revealed that majority of Form 4 students had negative attitude towards sexual behaviour. The study recommends that the Teachers Service Commission and other stakeholders should motivate teachers who teach HIV/AIDS education in secondary schools.