The Kyzylsu (Russian: Кызылсу) is a river that rises on the southern slopes of the Vakhsh Range in the north-east of Tajikistan's Khatlon Region and runs south-west until joining the Panj on the border with Afghanistan. The river is 230 kilometres (140 mi) long and has a basin area of 8,630 square kilometres (3,330 sq mi). It merges with the Yakhsu as a major left tributary south of the town of Kulyab. It irrigates the cotton-growing Kyzylsu Valley between Kulyab and Panj in the south-east of Khatlon Province. It is not the Kyzyl-Suu River that rises in Kyrgyzstan and flows through Tajikistan as Surkhob, then Vakhsh, following a course north-west of Kyzylsu.