Southern Maryland, also referred to as SoMD, is a geographical, cultural and historic region in Maryland composed of the state's southernmost counties on the Western Shore of the Chesapeake Bay. According to the state of Maryland, the region includes all of Calvert, Charles, and St. Mary's counties and the southern portions of Anne Arundel and Prince George's counties. It is largely coterminous with the region of Maryland that is part of the Washington metropolitan area. Portions of the region are also part of the Baltimore Metropolitan Area and the California-Lexington Park Metropolitan Statistical Area. As of the 2020 Census, the region had a population of 373,177. The largest community in Southern Maryland is Waldorf, with a population of 81,410 as of the 2020 Census.

Initially inhabited by the indigenous Piscataway people, the first European settlement in Southern Maryland (and the state as a whole) was established in St. Mary's City in 1634. While the settlement was intended to be a Catholic refuge, religious strife was prominent in Maryland's early years. The passage of the Maryland Toleration Act in 1649 resulted in St. Mary's City being cited as the birthplace of religious freedom in North America. The area developed an agricultural economy based on tobacco with labor sourced from indentured servants and slaves. The War of 1812 saw military action in the region during the British campaign to capture Washington, while the American Civil War saw the end of slavery in Maryland and John Wilkes Booth fleeing through the region following the Assassination of Abraham Lincoln. Much of the area remains rural with agriculture still playing a large role in the region's economy. Despite this, the region has seen large suburban growth and development in the late 20th and early 21st centuries as urban sprawl from nearby Washington D.C. expands southward.

In 2023, the region was designated as the Southern Maryland National Heritage Area.

Counties located in Southern Maryland include Calvert County, Charles County, St. Mary's County, and the southern portions of Prince George's County and Anne Arundel County. The region's northern boundary passes through Prince George's County and Anne Arundel County, east of Washington. Its eastern boundary is the Chesapeake Bay and its southern and western boundary is the Potomac River, Maryland's boundary with Virginia (and through it, the Northern Neck).

The Patuxent River runs through Southern Maryland, separating Calvert County and Anne Arundel County from Charles County, Prince George's County and St. Mary's County.

Land features in Southern Maryland include the St. Mary's Peninsula and the Calvert Peninsula.

Geologic formations of the Chesapeake Group can be found in Southern Maryland, including the Calvert Formation, the St. Marys Formation, and the Choptank Formation. Many of these formations are present at the Calvert Cliffs State Park in Calvert County.

Southern Maryland is within the Atlantic Coastal Plain physiographic region.