The Bōsō Peninsula lies east of Tokyo and forms the eastern side of Tokyo Bay. It occupies almost all of Chiba Prefecture on Honshu, Japan’s largest island, and separates the bay from the Pacific Ocean.
Its geography is defined by Tokyo Bay to the west, the Pacific Ocean to the east and south, and the Edo and Tone rivers to the north. The peninsula covers approximately 5,034 square kilometres, with urban and industrial areas concentrated in the north and west and rice-growing valleys and coastal farmland farther inland.
The name Bōsō reflects the historical provinces that occupied the region: Awa, Kazusa, and Shimōsa. The Japan Meteorological Agency distinguishes the bay-facing western coast as Uchibō and the Pacific-facing eastern coast as Sotobō, terms still used for local geography and travel.
The peninsula is also tied closely to modern Tokyo. The Tokyo Bay Aqua-Line, a bridge-and-tunnel route, connects Kisarazu on Bōsō with Kawasaki across the bay. Mount Atago, rising to 408.2 metres, is its highest point.