Onsu County and from Mandarin Chinese Wensu County, is a county in the Xinjiang Uyghur Autonomous Region and is under the administration of the Aksu Prefecture, bordering Kyrgyzstan's Issyk-Kul Region to the northwest. It contains an area of 14,309 km2 (5,525 sq mi).

Aksu Prefecture is located in mid-Western Xinjiang, China. It has an area of 131,161 km2 (50,642 sq mi) and 2.714 million inhabitants at the 2020 census whom 715,319 lived in the built-up area made up of Aksu urban district. The name Aksu is Turkic for 'white water'. Aksu Prefecture has a 263.8 km (163.9 mi) long international boundary with Kyrgyzstan and Kazakhstan.