Atlantic County is a county in the southern part of the U.S. state of New Jersey. As of the 2020 census, the county was the state's 15th-most-populous county, with a population of 274,534, a drop of 15 from the 2010 census count of 274,549. The United States Census Bureau's Population Estimates Program estimated a 2024 population of 279,114, an increase of 4,580 (+1.7%) from the 2020 decennial census. Its county seat is the Mays Landing section of Hamilton Township. The county is part of the Jersey Shore region of the state.

The most populous place in Atlantic County is Egg Harbor Township, with 47,842 residents at the time of the 2020 census; Galloway Township, covered 115.21 square miles (298.4 km²), the largest total area of any municipality, though Hamilton Township has the largest land area, covering 111.13 square miles (287.8 km²). Atlantic County forms the Atlantic City–Hammonton metropolitan statistical area, which is also part of the Philadelphia metropolitan area.

The county was named after the Atlantic Ocean, which borders the county's eastern coast.

Since the 6th millennium BC, Native American people have inhabited New Jersey. By the 17th century, the Absegami tribe of the Unalachtigo Lenape tribe – "people near the ocean" – stayed along the streams and back bays of what is now Atlantic County. The group referred to the broader area as Scheyichbi – "land bordering the ocean". European settlement by the Dutch, Sweden, and England contributed to the demise of the indigenous people. In 1674, West Jersey was established, and its provincial government designated the court of Burlington County in 1681, splitting off Gloucester County five years later from the southern portion. This county was bounded by the Mullica River to the north, the Atlantic Ocean to the east, and the Great Egg Harbor River and Tuckahoe River to the south. Great Egg Harbour Township, also called New Weymouth and later just Egg Harbor, was designated in 1693 from the eastern portions of Gloucester County.

The region's early European settlers, many of them Quakers, lived along the area's waterways. In 1695, John Somers purchased 300 acres (120 ha) of land on the northern shore of the Great Egg Harbor Bay in 1695, the same year he began ferry service across the bay to Cape May County. His son, Richard, built Somers Mansion between 1720 and 1726, which is the oldest home in existence in the county. Daniel Leeds first surveyed the coastal waters of Egg Harbor in 1698, eventually finding Leeds Point. In 1735, according to folklore, Mother Leeds gave birth to her 13th child in the area during a strange thunderstorm; this child would become the Jersey Devil. In the early 18th century, George May founded Mays Landing.

In 1774, the northern portion of Egg Harbor Township became Galloway Township. In 1785, residents in what is now Atlantic County requested to split from Gloucester County to the New Jersey legislature, wanting a local court. Mays Landing – the region's largest community at the time, had more saloons than churches. Criminals could escape custody before reaching Gloucester City on a four-day wagon ride. In 1798, the western portion split off to become Weymouth Township, and in 1813, the northwestern portion partitioned to become Hamilton Township. On February 7, 1837, the New Jersey legislature designated Atlantic County from Galloway, Hamilton, Weymouth, and Egg Harbor townships, choosing Mays Landing as the county seat. In the same year, the Board of Freeholders was established as the county government. As of the 1830 census, the townships making up Atlantic County only had a population of 8,164, making it the least populated New Jersey county. By that time, a continuous line of houses extended from Somers Point to Absecon.

Mullica Township was established from Galloway Township in 1837. In 1852, Dr. Jonathan Pitney recommended Absecon Island as a health resort, and formed the Camden and Atlantic Railroad to construct the line from Camden to the coast. The company purchased land from Atlantic and Galloway Townships in 1853, then promoted and sold the lots. Atlantic City formed on May 1, 1854, in advance of the rail line opening on July 4 of that year. In 1858, Egg Harbor City was formed from portions of Galloway and Mullica townships. In 1866, Hammonton was founded from Hamilton and Mullica townships. A year later, portions of Hamilton Township split off to become Buena Vista Township. In 1872, Absecon was split from portions of Egg Harbor and Galloway townships. By 1885, more than half of the county's population lived in Atlantic City, and by 1910 this more than two-thirds of the county lived there.

With more people moving to the area in the late 1800s into the early 1900s, several municipalities were created in short succession – Margate City (then called South Atlantic City) in 1885, Somers Point in 1886, Pleasantville and Linwood in 1889, Brigantine in 1890, Longport in 1898, Ventnor in 1903, Northfield and Port Republic in 1905, and Folsom in 1906. On May 17, 1906, the eastern coastal boundary of Atlantic County was established. The final municipalities in the county to be created were Corbin City from Weymouth Township in 1922, Estell Manor from Weymouth Township in 1925, and Buena from Buena Township in 1948. In 1938, the county's western border was clarified with Camden and Burlington counties using geographic coordinates. After a peak in prominence in the 1920s during the prohibition era, Atlantic City began declining in population in the 1950s as tourism declined. The county's growth shifted to the mainland.