Background: Syndemics of illicit drug use and HIV remain as significant public health issues around the world. There has been increasing concern regarding the rapidly growing market of new psychoactive substances, particularly in Asia. In response, the Japanese government has increasingly banned such substances in recent years. We sought to identify the prevalence and correlates of use of quasi-legal psychoactive substances among people living with HIV/AIDS (PLHIV) in Japan. Methods: Data were derived from a nationwide survey of PLHIV conducted at nine leading HIV/AIDS care hospitals between July and December 2013. The prevalence and correlates of the use of quasi-legal psychoactive substances (e.g., synthetic cannabinoids, cathinone derivatives, etc. that had not been prohibited from using at the time of survey) among male participants were examined using multivariate survey logistic regression. Results: Among 963 study participants, the majority (95.3%) were male. The most commonly used drug among men was quasi-legal psychoactive substances (55.3% ever and 12.8% in the previous year). In multivariate analysis, the lifetime use of tryptamine-type derivatives (i.e., 5-MeO-DIPT or N,N-diisopropyl-5-methoxytryptamine) (adjusted odds ratio [AOR]: 2.42; 95% confidence interval [CI]: 1.36-4.28) and methamphetamine/amphetamine (AOR: 3.59; 95% CI: 2.13-6.04) were independently associated with recent quasi-legal psychoactive substance use. Conclusions: In our sample of male PLHIV in Japan, quasi-legal psychoactive substances were the most commonly used drugs. Individuals who had ever used tryptamine-type derivatives or methamphetamine/amphetamine were more likely to report recent quasi-legal psychoactive substance use, suggesting a potential shift in drug use patterns from regulated to unregulated substances among this population. These findings indicate a need for further research to examine implications for HIV care.