While early sexual debut is highly prevalent in Nepal, its link to sexually transmitted infections (STIs/HIV) risk factors has not been explored at a national level. The objective of this study was to assess potential association between early sexual debut and risk factors for STIs/HIV acquisition, including sexual risk behaviors, sexual violence, and teenage pregnancy among adults in Nepal. Data were taken from the nationally representative Nepal Demographic Health Survey (2011), which employed a two-stage complex design to collect data. A sample of 12,756 adults (ages 15–49 years) were included. Multivariate logistic models were conducted, adjusted for demographic characteristics, to assess the association between early sexual debut and STIs/HIV-related risk factors. The prevalence of early sexual debut in this sample was 39.2 %, with a mean age of coital debut at 17.9 years. After adjusting for potential confounders, individuals with early sexual debut were significantly more likely to report a history of STIs (aOR 1.19; 95 % CI 1.06–1.35) and had a significantly higher risk profile, including having multiple sex partner (aOR 2.14; 95 % CI 1.86–2.47), inconsistent condom use (aOR 0.72; 95 % CI 0.61–0.86), paid for sex (aOR 1.61; 95 % CI 1.14–2.27), a history of sexual violence (aOR 1.99; 95 % CI 1.63–2.43), and teenage pregnancy (aOR 12.87; 95 % CI 11.62–14.26). Individuals who have early sexual debut are more likely to engage in risk behaviors that place them at increased risk of STIs/HIV acquisition. STIs/HIV prevention strategies should aim at delaying sexual debut to decrease the disproportionate burden of adverse health outcomes, including STIs/HIV, among individuals in Nepal.