Mengzi is a city in the southeast of Yunnan Province, China. Administratively, it is a county-level city and the prefectural capital of the Honghe Hani and Yi Autonomous Prefecture, located about 243 kilometres (151 mi) southeast from Kunming, and 400 kilometres northwest from Hanoi, Vietnam. It is situated in the centre of a fertile valley basin on the Yunnan-Guizhou Plateau 1,310 metres (4,300 ft) above the sea level and was home to about 590,300 inhabitants as of 2021 census. Mengzi was formerly Mengzi County (蒙自县) until October 2010, when it was upgraded to a county-level city. Mengzi is the core area of South Yunnan Central City Cluster, which is officially regarded as the political, economic, cultural, and military centre of South Yunnan.