Understanding predisposing factors for sexually transmitted infections (STIs) in young adults may identify targets for public health interventions. We conducted a cross-sectional web-based survey of university students’ sexual attitudes, behaviours and lifestyles and self-reported rates of STI. A total of 827 students responded, 22.4% had two or more sexual partners in the previous year with inconsistent condom use and the lifetime prevalence of STIs was 9.6%. Factors associated with a diagnosis of STI were increasing age and number of sexual partners ever, female gender (adjusted odds ratio [AOR] 2.70, 95% confidence interval [CI] 1.31, 5.56) and use of crack (AOR 10.45, 95% CI 1.46, 75.16). For female students, these were increasing age and number of partners ever, whereas for male students having sex with other men (bisexual AOR 4.8, 95% CI 1.02, 22.595, homosexual AOR 17.66, 95% CI 3.03, 103.04) and use of crack (AOR 32.24, 95% CI 3.33, 312.08). Multiple partners and recreational drug use may predict incidence of STI. Prevention strategies need to aim at reducing risk behaviour across various activities.