Macomb is a city in and the county of McDonough County, Illinois, United States. It is situated in western Illinois southwest of Galesburg. The population was 18,588 at the 2000 census. Macomb is the home of Western Illinois University.
Macomb is located at 40°27′38″N 90°40′27″W / 40.46056°N 90.67417°W / 40.46056; -90.67417 (40.460501, -90.674048). The East Fork La Moine River flows past the northern part of the city.
According to the United States Census Bureau, the city has a total area of 10.2 square miles (26.5 km²). 9.9 square miles (25.5 km²) of it is land and 0.4 square miles (1.0 km²) of it (3.90%) is water.