BACKGROUND Though research findings have indicated that family characteristics have a bearing on sexual behavior, there is a paucity of published literature on the role of family functionality and parental characteristics on adolescent sexual decision-making. This study was designed to assess the role of family function and parental influence on sexual behavior of in-school youths in secondary schools in a sub-urban area of Southwest Nigeria. METHODS A descriptive cross-sectional study was conducted among secondary school students using semi-structured interviewer-assisted questionnaire. Information was obtained on the sexual behavior and parent-child characteristics. Family functionality was assessed using family APGAR standardized instrument. Association was established using χ2-test for qualitative variables and t-test for quantitative variables at p=0.05. RESULTS Mean age of respondents was 14.8±2.2 years. Majority were from monogamous family setting (70.7%) and lived with their families (75.6%). About a quarter (26.8%) was from dysfunctional families and 9.2% had ever had sexual intercourse. Recent sexual engagement in the preceding 1 month was reported (47.4%). The mean score for parental monitoring, father-child communication, mother-child communication, and parental disapproval of sex were 10.4±2.2, 9.3±2.3, 9.8±2.4, and 10.4±2.3, respectively. There was a significant association between parental monitoring (t=3.9, p≤0.001), mother-child communication (t=3.03, p=0.003), and parental disapproval of sex (t=5, p≤0.001); and sexual experience. CONCLUSION This study showed that parental influence had a vital role in the sexual behavior of young persons. Advocacy and health education interventions are needed among parents regarding their role in the sexual behavior of in-school youths.