Objective: This study aimed to assess the relationship between knowledge of antiretroviral therapy (ART) and stigma regarding adherence to ART. Method: This cross-sectional study used a purposive sampling technique and involved 175 men who have sex with men (MSM), all of whom have HIV/AIDS. Levels of knowledge, stigma, and adherence were measured using the ART knowledge instrument, the Berger HIV Stigma Scale, and the Medication Adherence Questionnaire (MAQ), respectively. Results: Most of the respondents had a high level of ART knowledge (76.6%), HIV-related stigma (51.4%), and non-adherence to ART (52%). The study showed a statistically significant negative correlation between stigma and adherence to ART (p = 0.043; α = 0.05). In addition, ART knowledge correlated positively with adherence to ART (p = 0.010; α = 0.05). Regression analysis showed that ART knowledge is an important factor influencing ART adherence (OR = 2.817). Conclusions: It was found that as the level of ART knowledge increased, self-motivation and attitudes toward adherence to treatment increased.