Kyzylsu River rises on the southern slopes of the Vakhsh Range in the north-east of Tajikistan's Khatlon Province and runs south-west for a length of 230 km until joining Panj River on the border with Afghanistan. Merges with Yakhsu River as a major left tributary south of the town of Kulyab. Irrigates the cotton-growing Kyzylsu Valley between Kulyab and Panj in the south-east of Khatlon Province. Not to be confused with Kyzyl-Suu River, which rises in Kyrgyzstan and flows through Tajikistan as Surkhob and then Vakhsh River, following a course north-west of Kyzylsu.