The Koito River (小糸川, Koito-gawa) is a river in Futtsu and Kimitsu, Chiba Prefecture, Japan. The river is 82 kilometers (51 mi) in length and has a drainage area of 148.7 square kilometers (57.4 sq mi). The Koito is the third-longest river in Chiba Prefecture, and under the Rivers Act of 1906 the Koito is designated as a Class 2 River. The name of the river in Japanese is a combination of two kanji characters, the first meaning “little”, and the second meaning “thread”.