A rapidly emerging and highly concentrated HCV outbreak has recently been observed among both acute and chronic HIV-positive MSM in Bangkok, Thailand. NS5B regions of the HCV genome were amplified using nested-PCR and sequenced. Phylogenetic inference was constructed by Maximum Likelihood methods and clusters were identified with support and genetic distance thresholds of 85% and of 4.5%. Forty-eight (25 acute HIV and 23 chronic HIV) MSM with incident HCV infection were included in the analysis. HCV genotype(GT) was 85% GT 1a and 15% GT 3a or 3b. Median age at HCV diagnosis was 34(IQR, 28-41) years. 83.3%(40/48) had history of syphilis infection and 36%(16/44) reported crystal methamphetamine use. Only 2(4%) reported ever injecting drugs, both crystal methamphetamine. In the phylogenetic clustering analysis, 83% belonged to one of two clusters: one large(75%) and one small(8%) cluster. All clusters were GT 1a. MSM with acute HIV infection were more likely to be in a cluster(92%) than those with chronic infection(74%). HCV screening should be regularly performed for MSM in ART clinics, and offering direct-acting antiviral agents to all MSM with HCV infection might contain the HCV epidemic from expanding further.This article is protected by copyright. All rights reserved.