Bōsō Peninsula (房総半島 Bōsō-hantō) is a peninsula in Chiba prefecture on Honshū, the largest island of Japan. It forms the eastern edge of Tokyo Bay, separating it from the Pacific Ocean.
Most of the peninsula is hilly, with some points as high as 400 metres (1,300 ft) above sea level. While the western part of the peninsula is highly urbanized (centered on the city of Kisarazu), the other coastal lowlands and interior river valleys are chiefly used for growing rice.
The Tokyo Bay Aqua-Line, a bridge-tunnel across Tokyo Bay, connects Kisarazu with the city of Kawasaki in Kanagawa Prefecture.
The peninsula gets its name and kanji from the former provinces that were located there: Awa (安房), Kazusa (上総) and Shimōsa (下総).