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is a borough in Armstrong County, Pennsylvania, United States, 25 miles (40 km) northeast of Pittsburgh in a former coal-mining region. Apollo was settled in 1790, laid out in 1816, and incorporated as a borough in 1848. The town was originally known as 'Warren', but was later renamed to avoid confusion with another town in Pennsylvania of the same name.
The population in 1890 stood at 2,156; in 1900, 2,924; in 1910, 3006; in 1930, 3,406. The population was 1,765 at the 2000 census.
According to the United States Census Bureau, the borough has a total area of 0.3 square miles (0.9 km²), of which, 0.3 square miles (0.8 km²) of it is land and 0.04 square miles (0.1 km²) of it (8.57%) is water.