Background: Methamphetamine is associated with both injection and sexual risk behaviors. Methamphetamine use has increased significantly in Vietnam in the last 5-10 years. We examine effects of increased methamphetamine use on an existing HIV epidemic among heterosexual persons who inject drugs (PWID) in Haiphong, Vietnam, with specific focus on sexual transmission of HIV. Methods: Using respondent driven sampling (RDS), we recruited persons currently injecting drugs. Participants were administered a quantitative questionnaire on risk behaviors and tested for HIV and hepatitis C (HCV). Bivariate and multivariable logistic regression assessed associations between current methamphetamine use, injection and sexual risk behaviors. Results: We recruited 1336 heterosexual PWID in 2016-2017; they were predominately male (94%), injected heroin (~100%) and 51% reported current methamphetamine use. In the previous six months, among those sexually active (54%), 71% reported unsafe sex with any partners and among those with primary partners (45%), 79% reported unsafe sex with primary partners. In multivariable analysis, current methamphetamine use among HIV seropositives was associated with any unsafe sex (AOR 2.30, 95% CI: 1.20-4.42) and unsafe sex with primary partners (AOR 2.43, 95% CI: 1.15 - 5.10). Current methamphetamine use was not associated with unsafe sex among HIV seronegatives or with injection risk behavior. Conclusions: The association with current methamphetamine use and potential sexual transmission of HIV in Haiphong is of considerable concern. Methamphetamine increases the likelihood of PWID transmitting HIV to non-injecting primary sexual partners and cam reduce adherence to ART leading to immune system exhaustion. All participants were injecting heroin; methamphetamine use may interfere with opiate use treatment. Public health efforts should focus on the increase in methamphetamine use and associated sexual risk behaviors. Emphasis on sexual risk behavior, particularly among those that are HIV positive, in conjunction with continued monitoring of ART adherence and HIV viral loads will be critical.