Highlights• Wastewater treatment and discharge account directly for 12 per cent and 4 per cent of global methane and nitrous oxide emissions, respectively. In addition, drinking water and wastewater management are responsible for approximately 4 per cent of global electricity consumption, often associated with indirect carbon emissions. It is expected that, by 2030, the amount of energy consumed will increase by 50 per cent. • Reducing the release of these greenhouse gases (GHGs) is a major opportunity for climate change mitigation. Release of GHGs from wastewater and faecal sludge can be reduced through the improved design, management, and adjustment of operating conditions of wastewater treatment plants (WWTPs). Similarly, energy efficiency measures and other solutions (e.g., increased use of renewable energies) can be implemented to decrease energy consumption and reduce carbon dioxide (CO2) emissions. • A significant proportion of the wastewater generated in cities and rural areas remains untreated or only partially treated, with the emissions from untreated wastewater being three times higher than emissions from conventional WWTPs. In addition, millions of people currently have limited or no access to sanitation, and the mitigation potential of providing them with access to safely managed sanitation services cannot be underestimated. The extension of wastewater collection and treatment systems, including decentralized solutions, emerges as a win-win for development and climate mitigation.• Water utilities are increasingly measuring and reporting their GHG emissions and savings as part of national GHG inventories, using tools such as the publicly available Energy Performance and Carbon Emissions Assessment and Monitoring (ECAM) tool. However, there is a need to strengthen assessment, monitoring, and reporting of GHG emissions from water and wastewater handling, including on-site sanitation. The actual mitigation potential is largely unknown because data on GHG emissions is limited and has high levels of uncertainty. • This data and knowledge gap hampers effective integration of water, sanitation, and hygiene (WASH) in climate policies and mitigation strategies. It also presents a challenge to making climate finance available.