Background Globally, sexually transmitted infections are responsible for a high burden of morbidity and mortality and the complications of improperly managed infections are felt years after the event. It has also been linked to development of cancers. In sub-Saharan Africa, poverty, gender inequality and unemployment puts young females at risk. This study was carried out to assess STI and HIV knowledge, prevalence and related behaviour among young female traders in Mushin community Lagos, Nigeria

Methods A cross-sectional descriptive study was conducted using interviewer-administered questionnaires among female trader’s 15–24 years of age who sell wares in garages in an urban slum. Sample size of 290 was calculated. Simple random sampling was used to select divisions of garages. Data was analysed using Epi info version7, association was established using chi square at p < 0.05.

Results The mean age for the females was 21 ± 2.39. Majority were single (63.8%), had at least a secondary school education (82.8%) and had heard of STIs. The main source of information was friends/family (41.9%) and media (41.2%). Approximately half of the respondents had good knowledge regarding transmission, prevention, symptoms and types of STI. 13.8% reported symptoms of STI in the six months preceding the study mainly itching, discharge and painful urination. Most practiced self- medication (85.0%) and did not use condoms while having symptoms (80.0%). Less than 40% had been tested for HIV, mainly to know their status. Among sexually active traders 32.4% tested with their main partner and 44.9% know the HIV status of their sexual partners. Married traders (p = 0.002), older females (0.009) and those with at least a secondary school education (p < 0.000) were more likely to have been tested for HIV.

Conclusion Young female traders in this study had poor knowledge, high prevalence of STI and engage in risky behaviour.