South Africa is home to the largest number of people living with HIV/AIDS in the world. Alongside the HIV/AIDS epidemic, problematic alcohol and other drug (AOD) use is prevalent and associated with poor HIV treatment and secondary HIV prevention outcomes. International guidelines and local policy both support the integration of mental health care and AOD treatment into HIV care, yet barriers exist to implementation. This study aimed to explore patient and provider perspectives on the integration of HIV and AOD treatment services in Cape Town, South Africa. This included barriers and facilitators to task sharing AOD treatment in HIV care and preferences for a task shared approach to integrating AOD treatment in HIV care, including who should deliver the behavioural intervention. We conducted thirty semi-structured qualitative interviews with HIV and AOD treatment staff, providers, and people living with HIV/AIDS (PLWH) with moderate, problematic AOD use and difficulties (personal or structural) adhering to HIV treatment. Findings illustrated several key themes: (1) the separation between AOD and HIV services (a "siloed treatment experience"), even in the context of geographic co-location; (2) low AOD treatment literacy among HIV patients and providers, including a low awareness of existing AOD use services, even when co-located; (3) substance use stigma as a barrier to HIV and AOD treatment integration; (4) a strong patient preference for peer interventionists; and (5) the role of community health workers (CHWs) in detecting AOD use among some PLWH who had not followed up in HIV care. These findings will inform a future type 1 hybrid effectiveness-implementation trial, guided by the RE-AIM framework, to evaluate a task shared, evidence-based intervention to address problematic AOD use and improve HIV medication adherence in this setting.