Peatlands are globally‐important terrestrial carbon stores as well as regional sources of potable water supply. Water draining from peatlands is rich in dissolved organic carbon (DOC), which can be problematic for water treatment. However, it is unclear how future climate and sulfate deposition changes may impact DOC in peatland‐derived potable water. The United Kingdom (UK) is a global hotspot that consumes 79% of all potable water derived directly from peatlands. Here, a physically‐based hydrological model and a biogeochemical organic carbon model were used to predict discharge and DOC concentration in nine hotspots of peatland‐derived potable water use in the UK under a range of 21st century climate and sulfate deposition scenarios. These nine catchments supply 72% of all peatland‐derived water consumed in the UK and 57% of the global total, equivalent to the total domestic consumption of over 14 million people. Our simulations indicate that annual discharges will decrease and that mean annual DOC concentrations will increase under all future scenarios (by as much as 53.4% annually for the highest emissions scenario) in all catchments. Large increases (by as much as a factor of 1.6) in DOC concentration in the 2090s over the baseline period are projected for autumn and winter, seasons when DOC concentrations are already high in the baseline datasets such that water treatment works often reach their capacity to cope. The total DOC flux is largely insensitive to future climate change because the projected increase in DOC concentration is mostly counterbalanced by the projected decrease in discharge. Plain Language Summary Peatlands are important sources of potable water in some parts of the world. The UK is a particular hotspot and consumes around 79% of all drinking water provided by peatlands globally. Water draining from peatlands is rich in dissolved organic carbon (DOC). DOC from peatlands represents an important component of the global carbon cycle and is problematic for water treatment. Using physically‐based hydrological and organic carbon models combined with future climate and sulfate deposition scenarios for the UK, we demonstrate that river DOC concentrations are likely to increase under all future scenarios, particularly in autumn and early winter. These changes will create problems for water treatment because many water treatment plants that remove DOC already reach capacity during these seasons. Furthermore, large decreases in river discharge are projected in future summers for these important catchments, creating additional pressure for UK water resources. Key Points Mean annual DOC concentrations in nine UK peatland rivers will increase in the future, by as much as 53% for the highest emissions scenario Large increases in mean DOC concentrations are projected in future autumn and winter, periods when DOC concentrations are already high Large decreases in mean discharge are projected for April to September, periods when discharge is already low