The Ruak River (Thai: แม่น้ำรวก, RTGS: Maenam Ruak, Thai pronunciation: [mɛ̂ːnáːm rûək]) is a river at the Thai-Burma border. It originates within the hills of the Shan State (Burma), and becomes the boundary river between Thailand and Burma at the confluence with the Mae Sai River at the northernmost point of Thailand. It then meanders to the east until it empties into the Mekong River at Ban Sop Ruak, Tambon Wiang, Chiang Saen district, Chiang Rai Province. The boundary section of the river is 26.75 km long.