Background Human immunodeficiency virus (HIV) risky behaviours including multiple sexual partnership (MSP) and non-condom use (nCU) are known to be drivers of the spread of HIV; cognitive factors including perceived susceptibility of HIV, self-efficacy and attitudes play a significant role in influencing risky sexual behaviours. Objectives We sought to investigate personal beliefs, perceptions, thoughts and actions that are associated with MSP and nCU in South Africa. Methods We analysed nationally representative data from the 2012 National HIV Communication Survey (NCS) that included about 10 000 participants aged 16–55 years. Five constructs were created to measure psychosocial and cognitive determinants. Cronbach’s alpha coefficient for internal consistency reliability was calculated. Multivariable logistic regression was used to determine factors associated with MSP and nCU. Results Of the 6061 sexually active respondents, 13% (95% CI: 11.47–13.12) reported MSP and 52.7% ( n = 3158 of 6039) (95% CI: 51.0–53.55) nCU at last sex. Factors associated with MSP included perceived benefits, adjusted odds ratio (aOR) = 2.16 (95% CI: 1.80–2.58), perceived susceptibility to HIV, aOR = 2.22 (95% CI: 1.83–2.69) and engaging in intergenerational sex, aOR = 2.14 (95% CI: 1.78–2.56). Predictors of nCU were perceived benefits, aOR = 1.25 (95% CI: 1.09–1.43); perceived susceptibility to HIV, aOR = 1.6 (95% CI: 1.39–1.83); and personal beliefs, aOR = 1.35 (95% CI: 1.13–1.62). Conclusion Cognitive and behavioural factors were found to be predictors of risky sexual behaviours for HIV. This highlights the importance of considering personal perception and reasoning when attempting to understand and influence an individual’s sexual behaviour. This could be done through enhancing awareness of HIV risk in the general population and by influencing cognitive behaviour change through community mobilisation, advocacy and creating activities to improve self-esteem.