This research aimed to study the effect of premarital sex on sexually transmitted infections (STIs) and high risk behaviors among women in sub-Saharan Africa. It included 1393 women randomly selected from the Moshi urban district of northern Tanzania. Participants’ demographic and socio-demographic characteristics, alcohol use, condom use, number of partners, symptoms of STIs and age at first sex and marriage were obtained. Moreover, blood and urine samples were tested for HIV-1, HSV-2, syphilis, chlamydia, gonorrhea, trichomonas and Mycoplasma genitalium infections. The average duration of premarital sex in the study participants was 1.66 years (SD of 2.61 years). Women with longer duration of premarital sex had higher odds of HIV-1, HSV-2 and other STIs. Moreover, women with longer duration of premarital sex were more likely to report multiple sexual partners. These findings highlight the importance of a lengthy period of premarital sex as a public health issue. STIs prevention programs in sub-Saharan Africa should address factors leading to a longer period of premarital sex in women.