IMPORTANCE: Methamphetamine use is increasingly prevalent and associated with HIV transmission. A previous phase 2a study of mirtazapine demonstrated reductions in methamphetamine use and sexual risk behaviors among men who have sex with men. OBJECTIVE: To determine the efficacy of mirtazapine for treatment of methamphetamine use disorder and reduction in HIV risk behaviors. DESIGN, SETTING, AND PARTICIPANTS: This double-blind randomized clinical trial of mirtazapine vs placebo took place from August 2013 to September 2017 in an outpatient research clinic in San Francisco, California. Participants were community-recruited adults who were sexually active; cisgender men, transgender men, and transgender women who (1) had sex with men, (2) had methamphetamine use disorder, and (3) were actively using methamphetamine were eligible. Participants were randomized to receive the study drug or placebo for 24 weeks, with 12 more weeks of follow-up. Data analysis took place from February to June 2018. EXPOSURES: Mirtazapine, 30 mg, or matched placebo orally once daily for 24 weeks, with background counseling. MAIN OUTCOMES AND MEASURES: Positive urine test results for methamphetamine over 12, 24, and 36 weeks (primary outcomes) and sexual risk behaviors (secondary outcomes). Sleep, methamphetamine craving, dependence severity, and adverse events were assessed. RESULTS: Of 241 persons assessed, 120 were enrolled (5 transgender women and 115 cisgender men). The mean (SD) age was 43.3 (9.8) years; 61 (50.8%) were white, 31 (25.8%) were African American, and 15 (12.5%) were Latinx. A mean (SD) of 66% (47%) of visits were completed overall. By week 12, the rate of methamphetamine-positive urine test results significantly declined among participants randomized to mirtazapine vs placebo (risk ratio [RR], 0.67 [95% CI, 0.51-0.87]). Mirtazapine resulted in reductions in positive urine test results at 24 weeks (RR, 0.75 [95% CI, 0.56-1.00]) and 36 weeks (RR, 0.73 [95% CI, 0.57-0.96]) vs placebo. Mean (SD) medication adherence by WisePill dispenser was 38.5% (27.0%) in the mirtazapine group vs 39.5% (26.2%) in the placebo group (P = .77) over 2 to 12 weeks and 28.1% (23.4%) vs 38.5% (27.0%) (P = .59) over 13 to 24 weeks. Changes in sexual risk behaviors were not significantly different by study arm at 12 weeks, but those assigned to receive mirtazapine had fewer sexual partners (RR, 0.52 [95% CI, 0.27-0.97]; P = .04), fewer episodes of condomless anal sex with partners who were serodiscordant (RR, 0.47 [95% CI, 0.23-0.97]; P = .04), and fewer episodes of condomless receptive anal sex with partners who were serodiscordant (RR, 0.37 [95% CI, 0.14-0.93]; P = .04) at week 24. Participants assigned to mirtazapine had net reductions in depressive symptoms (Center for Epidemiologic Studies Depression Scale score, 6.2 [95% CI, 1.3-11.1] points lower; P = .01) and insomnia severity (Athens score, 1.4 [95% CI, 0.1-2.7] points lower; P = .04) at week 24. There were no serious adverse events associated with the study drug. CONCLUSIONS AND RELEVANCE: In this expanded replication trial, adding mirtazapine to substance use counseling reduced methamphetamine use and some HIV risk behaviors among cisgender men and transgender women who have sex with men, with benefits extending after treatment despite suboptimal medication adherence. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT01888835.