INTRODUCTION: Data on HIV antiretroviral therapy (ART) initiation among key-affected populations will support reaching the UNAIDS goal to end AIDS by 2030. METHODS: We assessed ART initiation among HIV-positive participants of the Bangkok Men Who Have Sex with Men (MSM) Cohort Study, which enrolled sexually experienced MSM aged ≥ 18 years and included visits every four months for a period of 3-5 years, from 2006-2016. At each visit, participants had HIV testing and completed computer-assisted self-interviewing on demographics and HIV risk behaviors. If they acquired HIV infection during the study, they received active referral for HIV treatment, continued in the cohort, and were asked about ART initiation. We used logistic regression to determine factors associated with ART initiation. RESULTS: Overall, 632 (36.2%) participants were diagnosed with HIV infection; 463 (73%) had a follow-up visit reporting information about ART, of those 346 (74%) reported ART initiation, with 323 (93%) on ART initiating ART through their registered national health benefit program. Only 70 (11%) were eligible for ART at time of diagnosis, and 52 (74%) initiated ART, on average, within six months of diagnosis. Multivariable analysis evaluating factors associated with ART initiation demonstrated that low CD4 cell count at time of diagnosis was the only independent factor associated with ART initiation. CONCLUSIONS: Most HIV-positive participants in the cohort reported ART initiation through the national health benefit program but limited data suggests there could be improvements in length of time to initiation of ART. Efforts should focus on ART start in MSM and transgender women soon after HIV diagnosis.