This paper reports on a study to compare the technological trajectory of drinking water treatment for the past 30 years, from the 1980s to the 2010s, in South Korea, Thailand, and Lao PDR. There are significant differences in water treatment technology in the three countries, resulting from factors such as national regulations, economic conditions, and water policies. These act either as drivers or as barriers for water service development in the three countries. South Korea has introduced various new treatment technologies since the 1990s for meeting stringent regulations and for improving tap water quality and safety. Thailand and Lao PDR maintained similar treatment methods without great changes, because they focused more on quantity expansion for meeting water demand than on quality improvement. In addition, lax regulations and financial constraints acted as barriers to the development of water treatment technology in the two countries. Thailand is now trying to apply new treatment technologies, such as online monitoring, mechanical sludge collector, and dual-media filter, as water quality has grown in importance, and Lao PDR has developed new water supply systems for increasing water supply coverage. This study explored changing patterns of treatment technology in the three countries, and offered lessons for developing water treatment technology in the region.