Ōnojō (大野城市, Ōnojō-shi) is an area located in Fukuoka Prefecture, Japan. Its name is made up of the kanji for 'big', 'field', and 'castle'. It is mostly a southern suburb of the city of Fukuoka, and has a border to the northwest with the Hakata-ku area of the city. It also shares borders with Dazaifu and Kasuga, Umi, Chikushino, Nakagawa and Shime. The border with Umi is marked by the summit of Otoganayama at 268 metres above sea level, a wide expanse of deciduous and bamboo forest, uninhabited except by the occasional snake. The Mikasa river flows through Ōnojō on its way to Hakata bay, with some of its tributaries such as Cow neck river originating in the mountains in the south.