John Abeel III (c. 1752–February 18, 1836) known as Gaiänt'wakê (Gyantwachia – "the planter") or Kaiiontwa'kon (Kaintwakon – "By What One Plants") in the Seneca language and thus generally known as Cornplanter, was a Seneca chief and diplomat. As a war chief, Cornplanter fought in the American Revolutionary War on the side of the British. After the war Cornplanter led negotiations with the United States and was a signatory of the Treaty of Fort Stanwix (1784), the Treaty of Canandaigua (1794), and other treaties. He helped ensure Seneca neutrality during the Northwest Indian War.

In the postwar years, Cornplanter endeavoured to learn more about Euro-American culture and invited Quakers to establish schools in Seneca territory. After the War of 1812 he became disillusioned by his people's poor reaction to Euro-American society. He had the schools closed and embraced his half-brother Handsome Lake's movement to return to traditional Seneca ways and religion.

The United States government granted him about 1500 acres of former Seneca territory in Pennsylvania in 1796 for "him and his heirs forever," which became known as the Cornplanter Tract. The federal government, however, started construction of the Kinzua Dam on the Allegany River in 1960. When the dam was completed the Cornplanter Tract was flooded and most of the few remaining residents moved to the Allegany Reservation of the federally recognized Seneca Nation of New York. Before flooding occurred the remains of Cornplanter, his descendants, and an 1866 monument to him were relocated.

Cornplanter was born about 1752 at Canawaugus (now in the Town of Caledonia) on the Genesee River in present-day New York State. He was the son of a Seneca woman, Gah-hon-no-neh (She Who Goes to the River), and a Dutch trader, Johannes "John" Abeel II.

The Dutch had settled the Hudson River Valley several generations earlier, and Cornplanter's father, an Albany fur trader, was part of an established family. The Abeel family name was sometimes Gaelicized to O'Bail and O'Beale. John Abeel II (1722–1794) was connected to the Schuyler family, leaders in business and politics. The grandfather after whom Cornplanter was named, Johannes Abeel I (1667–1711), was a trader and merchant who built up links with the indigenous people along his trade routes, and who served as the second mayor of Albany. The younger John Abeel was a gunsmith and was gladly welcomed into the Iroquois community to repair their guns.

Cornplanter was raised by his mother among the Seneca. His Seneca name, Gaiänt'wakê (often spelled Gyantwachia), means "the planter," and another variation, Kaintwakon, means "by what one plants." As the Seneca and other Iroquois nations had a matrilineal system of kinship, Cornplanter was considered a member of his mother's clan, the Wolf Clan.

When the American Revolutionary War began in 1775, Cornplanter urged the Seneca to remain neutral. He is reported to have said, "War is war. Death is death. A fight is a hard business."

Initially, both the British and the Americans also wanted the Iroquois to remain neutral. As the war progressed, however, the British began encouraging the Iroquois to "take up the hatchet," while the Americans sought their continued neutrality. In July 1777, the Seneca met with John Butler, a deputy superintendent in the British Indian Department at Irondequoit to discuss whether to abandon their neutrality. Although Cornplanter strongly opposed becoming involved, the Seneca eventually agreed to actively support the British against the Americans. Cornplanter honoured that decision.