Increased antiretroviral availability has decreased mortality and increased rates of comorbid chronic illnesses, including type 2 diabetes, among people living with HIV. Little work has compared within-person adherence rates for HIV and comorbid conditions. Sixty-two adults with HIV and type 2 diabetes reported adherence rates, illness representations, beliefs about medications, symptoms, side-effects, and negative mood states. Adherence to antiretrovirals was better than diabetes medication (95 vs. 90 %, z = -2.05, p = 0.04). Participants reported better control over diabetes compared to HIV (t = 1.98, p = 0.05) while antiretrovirals were considered more necessary than diabetes medication (t = -2.79, p < 0.05). In adjusted analyses, antiretroviral nonadherence was associated with antiretroviral concerns (OR = 0.24, 95 % CI 0.08-0.67) and diabetes medication nonadherence with diabetes-related symptom burden (OR = 0.69, 95 % CI 0.53-0.89). Results indicate that medication nonadherence varies within individuals across comorbid illnesses and suggest this variation may depend on symptom attribution and medication concerns.