Osaka is a designated city in the Kansai region of Honshu in Japan. It is the capital of and most populous city in Osaka Prefecture, and the thiā¦Osaka is a designated city in the Kansai region of Honshu in Japan. It is the capital of and most populous city in Osaka Prefecture, and the third-most populous city in Japan, following the special wards of Tokyo and Yokohama. With a population of 2.7 million in the 2020 census, it is also the largest component of the Keihanshin Metropolitan Area, which is the second-largest metropolitan area in Japan and the 10th-largest urban area in the world with more than 19 million inhabitants.