Yamanashi (yämäˈnäshē) [key], prefecture (1990 pop. 852,980), 1,724 sq mi (4,465 sq km), central Honshu, Japan. Kofu is the capital. The mountainous region is drained by the Fuji River. Yamanashi is a major producer of raw silk and fruit (grapes, peaches, apples, and cherries). Fuji-Hakone-Izu National Park is there.