Background Undisclosed antiretroviral drug (ARV) use among blood donors who tested HIV antibody positive, but RNA negative, was previously described by our group. Undisclosed ARV use represents a risk to blood transfusion safety. We assessed the prevalence of and associations with undisclosed ARV use among HIV‐positive donors who donated during 2017. Study design and methods South African National Blood Service (SANBS) blood donors are screened by self‐administered questionnaire, semi‐structured interview, and individual donation nucleic acid amplification testing for HIV. Stored samples from HIV‐positive donations were tested for ARV and characterized as recent/longstanding using lag avidity testing. Results Of the 1462 HIV‐positive donations in 2017, 1250 had plasma availability for testing of which 122 (9.8%) tested positive for ARV. Undisclosed ARV use did not differ by gender (p = .205) or ethnicity (p = .505) but did differ by age category (p < .0001), donor (p < .0001), clinic type (p = .012), home province (p = .01), and recency (p < .0001). Multivariable logistic regression found older age (adjusted odds ratio aOR 3.73, 95% confidence interval CI 1.98–7.04 for donors >40 compared with those <21), first‐time donation (aOR 5.24; 95% CI 2.48–11.11), and donation in a high HIV‐prevalence province (aOR 9.10; 95% CI 2.70–30.72) compared with Northern Rural provinces to be independently associated with undisclosed ARV use. Discussion Almost 1 in 10 HIV‐positive blood donors neglected to disclose their HIV status and ARV use. Demographic characteristics of donors with undisclosed ARV use differed from those noted in other study. Underlying motivations for nondisclosure among blood donors remain unclear and may differ from those in other populations with significant undisclosed ARV use.