The study investigated the motivation to disclose or the decision to withhold one’s HIV serostatus to one’s partners and assessed the implications of non-disclosure on young peoples’ sexual behaviour and access to treatment. This was a cross-sectional survey conducted with 253 youth aged 18–25 years receiving antiretroviral therapy in a health district in North West Province, South Africa. The majority were female (75%), the mean time since the HIV diagnosis was 22 months, 40% did not know their partner’s HIV status, 32% had more than two sexual partners, and 63% had not used a condom during the last sexual act. The prevalence of disclosure was 40%, 36% delayed disclosure for over a year, and most disclosed to protect the partner from HIV transmission, to receive support, and to be honest and truthful. The prevalence of non-disclosure was high, as 60% withheld disclosure due to fear of abandonment, stigma and discrimination, accusations of unfaithfulness, and partner violence. Over half (55%) had no intentions to disclose at all. The lower disclosure rates imply that HIV transmission continues to persist among sexual partners in these settings. The findings suggest that high levels of perceived stigma impact on disclosure and HIV treatment, which increases the risk of on-going HIV transmission among youth receiving long-term antiretroviral therapy (ART) in South Africa.