Arnold is a city in Westmoreland County, Pennsylvania in the United States. It is a suburb of Pittsburgh, part of the Pittsburgh Metro Area. The population was 5,667 at the 2000 census.
Around 1781, Robert McCrea purchased the land on which Arnold is situated. William Jack acquired the property and later passed it on to Wilson Jack. The area around Arnold was first settled in 1852 by Major Andrew Arnold, who grew up in Kittanning. Arnold was part of New Kensington until it was designated as a separate borough in January, 1896.
Arnold is located on the eastern shore of the Allegheny River, northeast of Pittsburgh. According to the United States Census Bureau, the city has a total area of 0.8 square miles (2.0 km²), of which 0.7 square miles (1.9 km²) is land and 0.1 square miles (0.1 km²) (6.41%) is water.