The county was established on March 7, 1891, with its county seat at Caldwell. Partitioned from Ada County, it originally included Payette County (1917) and the southern portion of Gem County (1915). Some sources attribute the name to the canyon of the Boise River near Caldwell, while western writers John Rees and Vardis Fisher believed it was named for the Snake River canyon, which forms a natural boundary with Owyhee County to the south and west.
