Pah-Ute County is a former county in the northwest corner of Arizona Territory, created from the division of the existing Mohave County on December 22 1865. Much of Pah-Ute County was in the small triangular section of what is now the southern part of the U.S. state of Nevada. The county seat was the town of Callville, which is now occupied by a part of Lake Mead. Pah-Ute was the historical spelling of the Indian tribe that is today referred to as the Paiute.
A short time later, on May 5 1866, all of Pah-Ute County west of the Colorado River and west of 114 degrees latitude became part of the new state of Nevada, over the protest of the territory of Arizona. The remaining portion of Pah-Ute County in Arizona was so underpopulated that no viable county government could be formed, and on February 18 1871, the remainder of Pah-Ute County was absorbed back into Mohave County.
Most of modern Clark County, Nevada, was once Pah-Ute County, Arizona, including Nevada's largest and most famous city, Las Vegas. To this day, some textbooks still refer to the Pah-Ute as Arizona's "Lost County".