Homosexuality-related stigma is associated with increased HIV risk behaviours among men who have sex with men (MSM) in low- and middle-income countries, but its impact on HIV treatment outcomes is less well understood. We examined the relationship between homosexuality-related stigma and failure to achieve sustained viral suppression among MSM on antiretroviral therapy (ART) in Hanoi, Vietnam. Between 2017 and 2020, 292 MSM were diagnosed with HIV in a prospective study. Socio-demographic, behavioural, homosexuality-related stigma, and HIV treatment data were collected. Multivariable Cox regression models examined associations between enacted, perceived, internalized, overall homosexuality-related stigma, and failure to achieve sustained viral suppression. Among 226 MSM on ART for ≥ 180 days (median age 24 years; median ART duration 23 months), 197 (87.2%) achieved sustained viral suppression. High overall stigma was reported by 86 men (38.1%), including enacted (26.1%), perceived (20.8%), and internalized (25.2%) stigma. After adjusting for potential confounders, no significant associations were found between enacted, perceived, internalized, or overall homosexuality-related stigma and sustained viral suppression. We observed a high level of homosexuality-related stigma and sub-optimal sustained viral suppression in this sample of MSM on ART. Targeted interventions are needed to decrease stigma and improve treatment outcomes for MSM living with HIV in Vietnam.