The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about 106,460,000 km² (41,100,000 sq mi). It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe, and Asia from the "New World" of the Americas in the European perception of the World.

Through its separation from Africa, Europe, and Asia from the Americas, the Atlantic Ocean has played a central role in the development of human society, globalization, and the histories of many nations. While the Norse were the first humans to cross the Atlantic, it was the 1492 expedition of Christopher Columbus that proved to be the most consequential. Columbus's expedition ushered in an age of exploration and colonization of the Americas by European powers, most notably Spain, France, Portugal, and the United Kingdom. From the 16th to 19th centuries, the Atlantic Ocean was the center of both an eponymous slave trade and the Columbian exchange while occasionally hosting naval battles. Such naval battles, as well as growing trade from regional American powers like the United States and Brazil, both increased in degree during the 20th century, and while no major military conflicts took place in the Atlantic in the present 21st century, the ocean remains a core component of trade across the world.

The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Europe and Africa to the east, and North and South America to the west. As one component of the interconnected World Ocean, it is connected in the north to the Arctic Ocean, to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, and the Southern Ocean in the south (other definitions describe the Atlantic as extending southward to Antarctica). The Atlantic Ocean is divided in two parts, by the Equatorial Counter Current, with the North(ern) Atlantic Ocean and the South(ern) Atlantic Ocean split at about 8°N.