City of Rocks National Reserve, National reserve and state park with granite formations in Cassia County, Idaho, United States

City of Rocks National Reserve is a 58.3 km² (14,400-acre) protected area in Cassia County, south-central Idaho, about 2 miles north of the Utah border. Its granite spires and monoliths belong to the Oligocene Almo pluton and Archean Green Creek Complex. A rectangular opening pierces one pale gray rock, and a long horizontal stone arm with a shallow fissure lies nearby.

Peter Skene Ogden's Snake River beaver trappers were the first Euro-Americans to note the rocks in 1826, but the area was ignored until 1843, when wagons began rolling through. The City of Rocks and nearby Granite Pass marked roughly halfway to California, and in 1852 some 52,000 people passed through on the way to the gold rush. Samuel J. Hensley's 1848 Salt Lake Cutoff rejoined the main trail here, and John Halley's stage line from Kelton, Utah, to Boise later used the same corridor.

The Shoshone and Bannock hunted bison in the area and gathered pinyon pine nuts; emigrant journals from the 1840s record smoke rising from the surrounding hills. The reserve's name comes from the emigrants, who thought the distant spires looked like a city; climbers now call it simply "The City".

The reserve holds more than 1,000 traditional and bolt-protected climbing routes, and climbers locally call the area "The City". Camp Rock was a favored campsite for emigrants and remains a place to camp. The reserve lies about 2 miles (3.2 km) north of the Utah border in Cassia County, off the route through Granite Pass.

The granite spires are part of the Oligocene Almo pluton, while the surrounding rock includes the much older Archean Green Creek Complex. Climbers in the region simply call the reserve "The City". In the 1980s Idaho climber Tony Yaniro developed some of the country's hardest routes here.