The water sector accounts for a significant proportion of the total energy consumption in urban areas; therefore, that sector can contribute to energy transition in urban areas. Seoul, South Korea has promoted the use of renewable energy and sewer heat as part of city-wide energy transition efforts. This study built energy consumption inventories for the urban water cycle in Seoul for 2012 and 2015 and investigated changes in net energy intensity and corresponding net carbon intensity during that period. It found that Seoul's energy transition efforts reduced net energy intensity in the water sector from 5.83 MJ/m(3) in 2012 to 5.42 MJ/m(3) in 2015, even with the increased use of energy-intensive advanced water treatment technology. In addition, this study estimated that about 8.52% of the water sector's current energy consumption could be saved in 2020 if 18.4 million m(3)/year of water were reused and 2.40 million m(3)/year of rainwater were harvested. This study showed a way to extend energy transition efforts into the urban water sector by reducing energy demand through reducing water demand.