Boise, the capital of Idaho and seat of Ada County, was given its name by the French-Canadian trappers who first explored the area. Astonished by the number of green cottonwood trees they found growing along the river, they labeled the area les bois ("the woods"). The land in and around Boise is still rich with trees, earning it the nickname "City of Trees."

The original Fort Boise was built at the mouth of the Boise River, about 40 miles from the city's present location. However, due to frequent raids by the Native Americans, the fort was abandoned in 1854. The discovery of gold set off a rush in the Boise Basin (in Idaho City) in 1862, and another fort was built. The new Fort Boise was erected in 1863 at the crossroads of the Oregon Trail and the road connecting the Boise Basin and Owyhee mining areas. A town soon sprung up around the fort, growing rapidly until it eventually surpassed Idaho City as the largest city in the area. Boise was officially incorporated as a city in 1864, and replaced Lewiston as the state capital in 1865.