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is the largest city in McClain County, Oklahoma, United States, and a part of the Oklahoma City Metropolitan Area. The population was 5,434 at the 2000 census.
According to the United States Census Bureau, the city has a total area of 52.9 square miles (137.1 km²), of which, 49.8 square miles (128.9 km²) of it is land and 3.2 square miles (8.2 km²) of it (6.01%) is water.
Newcastle is considered to be part of a rapidly growing area of northern McClain and Grady Counties known as the "Tri-City Area" with Tuttle and Blanchard.