BACKGROUND: People who inject drugs (PWID) are at elevated risk of HIV infection. Data on population sizes of PWID living with HIV are needed to inform the implementation of prevention, treatment and care programs. We estimated national population sizes of people who recently (past 12 months) injected drugs living with HIV and evaluated ecological associations with HIV prevalence in PWID. METHODS: We used national data on the prevalence of injecting drug use and of HIV among PWID, derived from systematic reviews, to estimate national population sizes of PWID living with HIV. Uncertainty was estimated using Monte Carlo simulation with 100,000 draws. We extracted data on sample characteristics from studies of HIV prevalence among PWID, and identified national indicators that have been observed or hypothesised to be associated with HIV prevalence in PWID. We used linear regression to evaluate associations between these variables and HIV prevalence in PWID. RESULTS: Four countries comprised 55% of the estimated global population of PWID living with HIV: Russia (572,500; 95% uncertainty interval (UI) 235,500-1,036,500); Brazil (462,000; 95% UI 283,500-674,500); China (316,500; 95% UI 171,500-493,500), and the United States (195,500; 95% UI 80,000-343,000). Greater anti-HCV prevalence and national income inequality were associated with greater HIV prevalence in PWID. CONCLUSION: The countries with the largest populations of PWID living with HIV will need to dramatically scale up prevention, treatment and care interventions to prevent further increases in population size. The association between anti-HCV prevalence and HIV prevalence among PWID corroborates findings that settings with increasing HCV should implement effective interventions to prevent HIV outbreaks. The association between income inequality and HIV among PWID reinforces the need to implement structural interventions alongside targeted individual-level strategies.