There are 23 counties and one independent city in the U.S. state of Maryland. Though formally an independent city rather than a county, the City of Baltimore is considered the equal of a county for most purposes and is functionally a county-equivalent in most respects. Many of the counties in Maryland were named for relatives of the Barons Baltimore, who were the proprietors of the Maryland colony from its founding in 1634 through 1771. The Barons Baltimore were Catholic, and George Calvert, 1st Baron Baltimore, originally intended that the colony be a haven for English Catholics, though for most of its history Maryland has had a majority of Protestants.

The last new county formation in Maryland occurred when Garrett County was formed in 1872 from portions of Allegany County. However, there have been numerous changes to county borders since that time, most recently when portions of the city of Takoma Park that had previously been part of Prince George's County were absorbed into Montgomery County in 1997.

Outside Baltimore (which is an independent city) the county is the default unit of local government. Under Maryland law, counties exercise powers reserved in most other states at the municipal or state levels, so there is little incentive for a community to incorporate. Many of the state's most populous and economically important communities, such as Bethesda, Silver Spring, Columbia, and Towson are unincorporated and receive their municipal services from the county. In fact, there are no incorporated municipalities at all in Baltimore County or Howard County. The county-equivalent is also the provider of public schools—school districts as a separate level of government do not exist in Maryland.

The City of Baltimore generally possesses the same powers and responsibilities as the counties within the state. It is an entity nearly surrounded by but separate from the County of Baltimore, which has its county seat in Towson.

The Federal Information Processing Standard (FIPS) code, which is used by the United States government to uniquely identify states and counties, is provided with each entry. Maryland's code is 24, which when combined with any county code would be written as 24XXX. The FIPS code for each county links to census data for that county.

County	FIPS code	County seat	Est.	Origin	Etymology	Flag	Seal	Population	Area	Map
Allegany County	001	Cumberland	1789	Formed from part of Washington County	From Lenape oolikhanna, which means "beautiful stream"			67,273	430 sq mi (1,114 km)
Anne Arundel County	003	Annapolis	1650	Formed from part of St. Mary's County	Anne Arundell was the maiden name of the wife of Cecil Calvert, 2nd Baron Baltimore. Between 1654 and 1658 it was known as Providence County by Puritan settlers			594,582	588 sq mi (1,523 km)
Baltimore County	005	Towson	1659	Formed from unorganized territory	Cecil Calvert, 2nd Baron Baltimore, first proprietor of the Maryland colony			844,703	682 sq mi (1,766 km)
Baltimore City	510	Baltimore City	1851	Founded in 1729. Detached in 1851 from Baltimore County	Cecil Calvert, 2nd Baron Baltimore, first proprietor of the Maryland colony			565,239	92 sq mi (238 km)
Calvert County	009	Prince Frederick	1654	Formed as Patuxent County from unorganized territory. Renamed Calvert County in 1658	The Calvert family; prior to 1658 it was called Patuxent County, after the Patuxent Indians, a branch of the Algonquians			94,728	345 sq mi (894 km)
Caroline County	011	Denton	1773	From parts of Dorchester County and Queen Anne's County	Lady Caroline Eden, daughter of Charles Calvert, 5th Baron Baltimore			33,593	326 sq mi (844 km)
Carroll County	013	Westminster	1837	From parts of Baltimore County and Frederick County	Charles Carroll of Carrollton, a representative to the Continental Congress and signatory of the Declaration of Independence			176,639	452 sq mi (1,171 km)
Cecil County	015	Elkton	1674	From parts of Baltimore County and Kent County	Cecil is an Anglicized form of the first name of Cecil Calvert, 2nd Baron Baltimore			105,672	418 sq mi (1,083 km)
Charles County	017	La Plata	1658	From unorganized territory	Charles Calvert, 3rd Baron Baltimore, second proprietor of the Maryland colony			171,973	643 sq mi (1,665 km)
Dorchester County	019	Cambridge	1668	From unorganized territory	Dorchester in Dorset, England; the Earl of Dorset was a friend of the Calvert family			32,879	983 sq mi (2,546 km)
Frederick County	021	Frederick	1748	From part of Prince George's County	Frederick Calvert, 6th Baron Baltimore, final proprietor of the Maryland colony			293,391	667 sq mi (1,728 km)
Garrett County	023	Oakland	1872	From part of Allegany County	John Work Garrett, president of the Baltimore and Ohio Railroad			28,423	656 sq mi (1,699 km)
Harford County	025	Bel Air	1773	From part of Baltimore County	Henry Harford, illegitimate son of Frederick Calvert, 6th Baron Baltimore			264,644	527 sq mi (1,365 km)
Howard County	027	Ellicott City	1851	From parts of Anne Arundel County and Baltimore County	John Eager Howard, an American Revolutionary War officer and governor of Maryland			336,001	254 sq mi (658 km)
Kent County	029	Chestertown	1642	From unorganized territory	The English county of Kent			19,303	414 sq mi (1,072 km)
Montgomery County	031	Rockville	1776	From part of Frederick County	Richard Montgomery, an American Revolutionary War general			1,058,474	507 sq mi (1,313 km)
Prince George's County	033	Upper Marlboro	1696	From parts of Calvert County and Charles County	Prince George of Denmark, the husband of Queen Anne of Great Britain			947,430	498 sq mi (1,290 km)
Queen Anne's County	035	Centreville	1706	From parts of Talbot County	Anne, Queen of Great Britain			52,508	510 sq mi (1,321 km)
Somerset County	039	Princess Anne	1666	From unorganized territory.	Mary, Lady Somerset, sister-in-law of Cecil Calvert, 2nd Baron Baltimore			24,910	611 sq mi (1,582 km)
St. Mary's County	037	Leonardtown	1637	From unorganized territory. Was named Potomac County between 1654 and 1658.	The Virgin Mary, first county named in a colony intended to be a haven for Catholics			115,281	611 sq mi (1,582 km)
Talbot County	041	Easton	1662	From part of Kent County	Grace, Lady Talbot, sister of Cecil Calvert, 2nd Baron Baltimore			37,823	477 sq mi (1,235 km)
Washington County	043	Hagerstown	1776	From part of Frederick County	George Washington, first President of the United States	None		155,813	468 sq mi (1,212 km)
Wicomico County	045	Salisbury	1867	From parts of Somerset County and Worcester County	The Wicomico River; in Lenape, wicko mekee indicated "a place where houses are built", possibly in reference to a settlement			104,800	400 sq mi (1,036 km)
Worcester County	047	Snow Hill	1742	From part of Somerset County	Mary Arundell, the wife of Sir John Somerset, son of Henry Somerset, 1st Marquess of Worcester, and sister of Anne Arundell, the wife of Cecil Calvert, 2nd Baron Baltimore			54,171	695 sq mi (1,800 km)

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County	Years of existence	Etymology
Old Charles County	1650–1654	King Charles I of England
Durham County	1669–1672	The English County Durham
Old Worcester County	1672–1685	Mary Arundell, the wife of Sir John Somerset, son of the 1st Marquess of Worcester, and sister of Anne Arundell, wife of Cecil Calvert, 2nd Baron Baltimore

• Maryland statistical areas
• List of ghost towns in Maryland