This paper assesses the potential greenhouse gas (GHG) emissions reduction impacts of urban water conservation. Using California as a case study, it estimates this co-benefit of California's statewide urban water conservation goal of 20% per capita reduction by 2020 (relative to a year 2000 baseline). We developed a model of a water supply system to assess the impact of reduced urban water demand on emissions. Embedded energy and emissions were established for each stage of the water supply cycle: supply and conveyance, treatment, distribution, end use and wastewater treatment. We conclude that water conservation, in addition to being an important strategy for adaptation to climate change, represents a significant opportunity for mitigation. Under policies that prioritize savings of water that is heated, the most energy-intensive process in the supply cycle, water conservation offers the potential to conserve 3.5 Mt CO2e in 2020. This result suggests that water conservation could be an important mitigation strategy in other states, even those that are not water-constrained and do not have highly energy intensive supply sources.