The study examined the influence of impulsivity on undergraduates’ sexual risk behaviour and also scrutinized the mediating roles of family type and peer pressure on undergraduates’ sexual risk behaviour in Southwestern Nigeria. The study was a descriptive survey that employed a multi-stage sampling procedure in which respondents were selected at different levels and stages. A total of 1080 respondents selected from six Universities participated in the study. Their age ranges from 15-19 years with a mean of 15.9±1.2 years. Data gathered through Impulsive Behaviour Scale and the Sexual Behaviour Inventory was analysed by t-test and ANCOVA. The result showed that impulsivity (t = 6.04, df =1078, p <0.05) had significant influence on the sexual risk behaviour of undergraduates in Southwestern Nigeria. The result further showed that age: F (2, 1077) = 18.20, p < 0.05; family type F (2, 1077) = 25.41, p <0. 05 and peer pressure F (2, 1077) = 53.13, p< 0.05 have an intervening impact on the influence of impulsivity F (2, 1073) = 1.37, p < 0.05) on sexual risk behaviour of the undergraduates. The study concluded that impulsivity enhances sexual risk behaviour of undergraduates in Southwestern Nigeria.