Shikoku (四国^?, "four provinces") is the smallest (225 km or 139.8 mi long and between 50 and 150 km or 31.1 and 93.2 mi wide) and least populous (4,141,955 as of 2005) of the four main islands of Japan, located south of Honshū and east of the island of Kyūshū. Its ancient names include Iyo-no-futana-shima (伊予之二名島), Iyo-shima (伊予島), and Futana-shima (二名島). The current name refers to the four former provinces which made up the island: Awa, Tosa, Sanuki, and Iyo.

The Shikoku region, comprising Shikoku and its surrounding islets, covers about 18,800 square kilometres (7,259 sq mi) and consists of four prefectures: Ehime, Kagawa, Kōchi, and Tokushima. Across the Inland Sea lie Wakayama, Osaka, Hyōgo, Okayama, Hiroshima, and Yamaguchi Prefectures on Honshū. To the west lie Ōita and Miyazaki Prefectures on Kyūshū.

The 50th largest island by area in the world, Shikoku is smaller than Sardinia and Bananal, but larger than Halmahera and Seram. By population, it ranks 23rd, having fewer inhabitants than Sicily or Singapore, but more than Puerto Rico or Negros.