Gay and bisexual men (GBM) report higher rates of methamphetamine use compared to heterosexual men, and thus have a heightened risk of developing problems from their use. We examined treatment outcomes among GBM clients receiving outpatient counseling at a lesbian, gay, bisexual, transgender and intersex (LGBTI)-specific, harm reduction treatment service in Sydney, Australia. GBM receiving treatment for methamphetamine use from ACON's Substance Support Service between 2012-15 (n = 101) were interviewed at treatment commencement, and after 4 sessions (n = 60; follow-up 1) and 8 sessions (n = 32; follow-up 2). At each interview, clients completed measures of methamphetamine use and dependence, other substance use, injecting risk practices, psychological distress and quality of life. The median age of participants was 41 years and 56.4% identified as HIV-positive. Participants attended a median of 5 sessions and attended treatment for a median of 112 days. There was a significant reduction in the median days of methamphetamine use in the previous 4 weeks between baseline (4 days), follow-up 1 (2 days) and follow-up 2 (2 days; p = .001). There was a significant reduction in the proportion of participants reporting methamphetamine dependence between baseline (92.1%), follow-up 1 (78.3%) and follow-up 2 (71.9%, p < .001). There were also significant reductions in psychological distress (p < .001), and significant improvements in quality of life (p < .001). Clients showed reductions in methamphetamine use and improved psychosocial functioning over time, demonstrating the potential effectiveness of a LGBTI-specific treatment service.