Tanacharisson (Half King) (1700? - 1754) was a leader of the Mingo band of the (Seneca tribe) of Native American Indians who took part in the French and Indian War as an ally of the British.

Tanacharisson was born into the Catawba tribe about 1700 near what is now Buffalo, New York. As a child, he was taken captive by the French and later adopted into the Seneca tribe, one of the Six Nations of the Iroquois Confederacy. His early years were spent on the southwestern shore of Lake Erie in what is now western New York state.

Tanacharisson was called Half King by the British colonists. Half King was actually a title rather than a proper name. In 1748, the Iroquois Confederacy named Tanacharisson the leader of the Seneca in the Ohio Country. Since these tribes were without a permanent council, he was designated a "half-king". As a half-king, he was given the power by the Iroquois Grand Council to conduct diplomacy with other tribes and to act as spokesman to the British.

By the early 1750s, Tanacharisson was living in the Seneca village of Logstown (present Ambridge, Pennsylvania) about 20 miles downstream from the forks of the Ohio River. The boundary between the British and French colonies in North America had never been clearly defined. Both sides claimed the Ohio River country making conflict between the two empires a near certainty.

In late 1753, the British discovered that the French had constructed a series of outposts in the Ohio River county. Robert Dinwiddie, the lieutenant governor of Virginia, ordered Lt. Colonel George Washington of the Virginia Militia to travel to the French outposts and demand that the French vacate the Ohio River country. On his way to the outpost, Washington’s party stopped at Logstown to ask Tahacharisson to accompany them as representative of the Seneca tribe. Tachacharisson traveled with Washington to meet with the French outpost at Fort Le Boeuf in what is now Franklin, Pennsylvania and Fort Presque Isle (present Erie, Pennsylvania). The French refused to vacate, setting the stage for conflict.

Tanacharisson accompanied George Washington in the spring of 1754 in an expedition to establish a fort at the strategic forks of the Ohio. Tanacharisson had advised the British to construct a “strong house” or fortress at the forks for the previous two years. As Washington approached the forks, he was horrified to discover that the French had beat him and constructed Fort Duquesne. Washington remained at his encampment at the Great Meadows, in what is now Fayette County, Pennsylvania awaiting reinforcements and further instructions. On May 27, 1754, Tanacharisson sent word to Washington that a patrol of French soldiers was approaching Washington’s camp. He urged the attack, arguing that the French force planned to ambush Washington.

The next morning, Washington, Tanacharisson, and a party of British soldiers surrounded the French soldiers and a fight broke out. In the fight, nine French soldiers were killed, 21 were captured, and two, including its commander Ensign Joseph Coulon de Jumonville, were wounded. Washington was meeting with the wounded Jumonville shortly after the battle discussing terms when Tanacharisson approached Jumonville and tomahawked and killed him saying, "Thou are not dead yet my father."

Some members of the French party had escaped to Fort Duquesne, informing the garrison of Washington’s actions. The French and British were not at war and Washington’s attack on the party of French soldiers would cause the French and Indian War to erupt. The commander of the Fort Duquesne, Jumonville’s half brother, Captain Louis Coulon de Villiers, mounted an expedition to drive the British out. The French accused Washington of an unprovoked attack on a French diplomatic party and charged Washington and Tanacharisson with assassinating Jumonville.

Washington erected a hastily constructed log fort named Fort Necessity at Great Meadows and awaited reinforcements. Washington asked Tanacharisson and another Seneca leader, Queen Alliquippa, to help defend the site against the French, but a quarrel over strategy erupted and Tanacharisson viewed Washington’s defenses as inadequate, calling Fort Necessity "that little thing upon the meadow." Tanacharisson moved his people east to the Susquehanna River valley and took no active part in the remainder of the war.

Tanacharisson died of pneumonia on October 4, 1754 on the farm of John Harris at Paxtang, Pennsylvania (near present-day Harrisburg, Pennsylvania).

Tanacharisson is spelled Tanaghrisson in some documents.