This study used data from the Bangladesh Adolescents Survey 2005 to identify socioeconomic and urban–rural determinants of knowledge regarding sexually transmitted infections (STIs) including HIV/AIDS transmission. A cluster sampling of 11 986 adolescents was conducted from April to August 2005. Data were analyzed using SPSS applying principle component analysis, multivariate logistic regression analysis, and prevalence ratios (PRs) with 95% confidence intervals (CIs). Overall knowledge of transmission of STIs was poor (4.5%), showing urban adolescents having twice the knowledge of rural adolescents (PR = 1.9; 95% CI = 1.6-2.2). HIV/AIDS knowledge level was high (68%), with a 40% higher knowledge among urban adolescents (PR = 1.4; 95% CI = 1.3-1.4). Probability of knowledge of STIs and HIV/AIDS transmission was lowest in 12- to 14-year-old uneducated female household workers of the poorest socioeconomic status in rural settings (0.0002 and 0.064, respectively). The urban–rural factor was more important than the socioeconomic factor. Health promotion and education programs can play an important role in improving the sexual reproductive health knowledge situation.