Abstract Introduction Detention settings are high-risk environments for the spread of infectious diseases. Since 2020, COVID-19 has posed unprecedented challenges for governments and prison administrations. In some jurisdictions, this has catalysed early release programmes to decongest prisons to minimise the harm of COVID-19 in prison systems. Methods From March to June 2020, HRI monitored the adoption of prison decongestion measures in response to COVID-19 in Europe and worldwide. HRI tracked criteria for eligibility and implementation of the measures and distributed online expert surveys as part of the Global State of Harm Reduction 2020 that included questions on harm reduction in prisons and the response to COVID-19. Survey data was supplemented by a review of academic, governmental, and non-governmental literature. A review to update the data will be carried out by mid-2021. Results and discussion Results show prison decongestion schemes initiated in 17 countries in Europe and 109 countries worldwide. Overall, by July 2020 decongestion measures reduced the global prison population by 16% in Europe and just 6% worldwide. In a quarter of countries (including at least four in Europe), people incarcerated for drug offences were explicitly excluded, regardless of whether they suffered from health condition or belong to a vulnerable group. We found no evidence of expanded access to harm reduction programmes to address the risk of overdose after release. Issues that exacerbate overdose risk included interruptions to the provision of opioid agonist therapy (OAT) and the unavailability in most jurisdictions of naloxone on release. Conclusions People who use drugs and are in detention settings have been inadequately served during the COVID-19 pandemic. To address the unique health risks of detention settings, there is a need for greater commitment to the adoption of non-custodial measures, and diversion from criminal justice towards a health-led response to drug use.