Yen Bai is a city in Vietnam. It is the capital of Yên Bái Province, in the north-east region of Vietnam. As of 2015 the district had a population of 99,850. Population calculating of Vietnam Government.
On 9 February 1930, part of the 4th Regiment of Tonkinese Rifles stationed at Yen Bai mutinied against their French officers in the Yên Bái mutiny. They were suppressed by loyal troops from the same unit. This incident led to widespread disturbances against French rule across northeastern Vietnam during 1930-31.