A harpoon is a long, spear-like projectile used in fishing, whaling, sealing, and other hunting to shoot, kill, and capture large fish or marine mammals such as seals, sea cows, and whales. It impales the target and secures it with barb or toggling claws, allowing the fishermen or hunters to use an attached rope or chain to pull and retrieve the animal. A harpoon can also be used as a ranged weapon against other watercraft in naval warfare.

Certain harpoons are made with different builds to perform better with the type of target. For example, the Inuit have short, fixed-foreshaft harpoons for hunting at breathing holes, while loose-shafted ones are made for throwing and remaining attached to the game.

Back in indigenous times, the indigenous peoples of Inuit and other alaska regions used a specialized hunting tool commonly called a “Walrus Harpoon” or an “Inuit Harpoon” to secure and kill mainly walruses and other marine animals. Unlike a simple spear, its most critical feature is a detachable head designed to toggle or turn sideways inside the animal's flesh, which prevents the line from pulling out. Walrus Harpoons are mainly sharp, long & pointy while usually being made with rawhide for the wooden shaft, ivory for the sharp spear head for easier killing, and usually an ice pick on standby for helping with getting animals out of tight spaces.

In the 1990s, harpoon points, known as the Semliki harpoons or the Katanda harpoons, were found in the Katanda region in Zaire. As the earliest known harpoons, these weapons were made and used 90,000 years ago, most likely to spear catfishes. However, this is disputed as the dating techniques used are less accurate at that epoch. Later, in Japan, spearfishing with poles was widespread in palaeolithic times, especially during the Solutrean and Magdalenian periods. Cosquer Cave in southern France has cave art over 16,000 years old, including drawings of seals that appear to have been harpooned.

There are references to harpoons in ancient literature, though in most cases the descriptions do not go into detail. An early example can be found in the Bible in Job 41:7 (NIV): "Can you fill its hide with harpoons or its head with fishing spears?" The Greek historian Polybius (c. 203 BC – 120 BC), in his Histories, describes hunting for swordfish by using a harpoon with a barbed and detachable head. Copper harpoons were known to the seafaring Harappans well into antiquity. Early hunters in India include the Mincopie people, aboriginal inhabitants of India's Andaman and Nicobar islands, who have used harpoons with long cords for fishing since early times.

In the novel Moby-Dick, Herman Melville explained the reason for the harpoon's effectiveness:

In most land animals there are certain valves or flood gates in many of their veins, whereby when wounded, the blood is in some degree at least instantly shut off in certain directions. Not so with the whale; one of whose peculiarities is, to have an entire non-valvular structure of the blood-vessels, so that when pierced even by so small a point as a harpoon, a deadly drain is at once begun upon his whole arterial system; and when this is heightened by the extraordinary pressure of water at a great distance below the surface, his life may be said to pour from him in incessant streams. Yet so vast is the quantity of blood in him, and so distant and numerous its interior fountains, that he will keep thus bleeding and bleeding for a considerable period; even as in a drought a river will flow, whose source is in the well springs of far off and undiscernible hills.

— Herman Melville, Moby-Dick, 1851