Grant County is a county located in the U.S. state of Washington. As of the 2020 census, the population was 99,123. The county seat is Ephrata, and the largest city is Moses Lake. The county was formed out of Douglas County in February 1909 and is named for U.S. President Ulysses S. Grant. Grant County comprises the Moses Lake, WA Micropolitan Statistical Area, which is also part of the Moses Lake-Othello, WA Combined Statistical Area.