Yanai is a city located in Yamaguchi Prefecture, Japan. As of 31 May 2023, the city had an estimated population of 29,821 in 15298 house…Yanai is a city located in Yamaguchi Prefecture, Japan. As of 31 May 2023, the city had an estimated population of 29,821 in 15298 households and a population density of 210 persons per km². The total area of the city is 140.05 square kilometres.