Detailed hydrogeology of the Chiang Rai and Pha Yao Provinces in northern Thailand was studied by the Department of Groundwater Resources, aiming for a proper groundwater management. Conjunctive use of surface water and groundwater is planned as a part of a green society development project. The area covers 11,000 square kilometers of alternating hills and plains that is divided into five basins, namely Mae Sai, Chiang Rai, Mae Suai, Wiang Pa Pao, and Phan–Pha Yao basins. The Mae Sai and Chiang Rai basins are located in the northernmost part of Thailand and is hydraulically connected to Mekong River and the adjacent areas of Myanmar and Lao PDR. The aquifers in the basins are characterized by both hard rocks and unconsolidated sediments. Included are granite, sandstone, limestone, and volcanic rock aquifers at the depth of 20-80 meters with a yield of up to 30 m3/hr, and the sand-gravel aquifers that may reach the depth of 250 meters with a yield of up to 50 m3/hr. Groundwater quality is generally characterized by high iron content, and in some places, by high manganese and fluoride contents. Groundwater flow direction, aquifer hydraulic properties, groundwater potential and groundwater uses are also assessed. This study provides a basic knowledge that benefits the Mekong River Commission (MRC) countries and also Dialogue Partners of the MRC, i.e. Cambodia, Lao PDR, Vietnam, Thailand, China and Myanmar. Joint management of their shared water and related resources would form a strong foundation for sustainable development and poverty alleviation in the Greater Mekong Subregion.