The parish has a total area of 2,115 km2 (816 mi2). 1,916 km2 (740 mi2) of it is land and 198 km2 (77 mi2) of it is water. The total area is 9.38% water.

Adjacent parishes
  • St. Landry Parish (north)
  • Pointe Coupee Parish (northeast)
  • Iberville Parish (east)
  • Iberia Parish †
  • Assumption Parish (southeast)
  • St. Mary Parish (southwest)
  • Lafayette Parish (west)

† St. Martin Parish boundaries are somewhat unique, and are not contiguous. The parish consists of two areas, separated by Iberia Parish.

The primary occupation is farming, with sugar cane being the dominant crop. The largest cities are Breaux Bridge, with 7,281 residents, and St. Martinville, the parish seat, with 6,989 residents.

The St. Martin Parish School District serves about 8,500 students. There are 16 public schools and 3 office complexes. There are also two Catholic schools, one Episcopal school, and one "Christian" school.

St. Martin Parish is the only parish in Louisiana that is geographically split into two separate areas. Upper St. Martin Parish contains 16 schools, while Lower St. Martin Parish contains only one school.

The city of St. Martinville is historically the original settlement of French Acadian people in Louisiana when they were expelled from Nova Scotia by the British in 1769. Thus, St. Martin Parish is truly the "Birthplace of Acadiana".