Mars is a borough in Butler County, Pennsylvania, United States. The population was 1,699 at the 2010 census.
Mars is located in southern Butler County at 40°41′48″N 80°0′44″W (40.696594, -80.012205), about halfway between the cities of Pittsburgh and Butler. The small community is nestled in a small valley along Breakneck Creek. Pennsylvania Route 228 bypasses the borough to the south, leading east 4 miles (6 km) to Pennsylvania Route 8 and west 5 miles (8.0 km) to Interstates 79 and 76 in Cranberry Township. The Mars-Evans City Road leads out of town to the north.
According to the United States Census Bureau, the borough of Mars has a total area of 0.5 square miles (1.2 km2), all land.
Mars is home to the popular roadside attraction the Mars "Spaceship" or "Flying Saucer". It is also home to the Mars Station, one of the last railroad depots still standing from the now defunct Pittsburgh and Western Railroad.
In 1873, Samuel Parks constructed a home and a water-powered gristmill along Breakneck Creek. Parks decided to have a post office placed in his home, so he received help from his friend Samuel Marshall to help establish it. The name of the post office became Overbrook. In 1877, the Pittsburgh, New Castle and Lake Erie Railroad was constructed through Overbrook, and had a station built there.