The study investigates the influence of family structure on adolescent sexual behaviour in Mushin Local Government Area of Lagos State, Nigeria. The research was conducted by taking a sample of 330 adolescents who are 15-24 years. The analytical techniques used in the study are: univariate, bivariate and chi-square tests. The key findings of this study are: firstly, there are two types of family structures identified which include: in-tact and divorced families. Secondly, that the in-tact (at p=0.000) has significant influence on adolescent sexual behaviour. Thirdly, at p=0.000, it was observed that divorced family has significant effect on adolescent sexual behaviour. The recommendations for this study are that there is urgent strong counsel for folks in Nigerian society at large to shun divorce and value marriages thereby having more intact-homes which will curtail to a large extent immoral sexual behaviour among the adolescents