Ishi (c. 1861 – March 25, 1916) was the last known member of the Native American Yahi people from the present-day state of California in the United States. The rest of the Yahi (as well as many members of their parent tribe, the Yana) were killed in the California genocide in the 19th century. Widely described as the "last wild Indian" in the United States, Ishi lived most of his life isolated from modern North American culture, and was the last known Native manufacturer of stone arrowheads. In 1911, aged 50, he emerged at a barn and corral, 2 mi (3.2 km) from downtown Oroville, California.

Ishi, which means "man" in the Yana language, is an adopted name. The anthropologist Alfred Kroeber gave him this name because in the Yahi culture, tradition demanded that he not speak his own name until formally introduced by another Yahi. When asked his name, he said: "I have none, because there were no people to name me", meaning that there was no other Yahi to speak his name on his behalf.

Anthropologists at the University of California, Berkeley, took Ishi in, studied him, and hired him as a janitor. He lived most of his remaining five years in a university building in San Francisco. His life was depicted and discussed in multiple films and books, notably the biographical account Ishi in Two Worlds published by Theodora Kroeber in 1961.

Ishi was likely born in the year 1861 within the heart of Yahi and Yana territory. At the time of Ishi's birth, the Yana were based in the Sierra Nevada Mountains area between the Pit and Feather Rivers, with the Yahi subgroup living in the southern portion. Written accounts from the 19th century suggest that the Yahi were hunter-gatherers who lived in small egalitarian bands without centralized political authority, chose to seclude themselves even from neighboring peoples, and fiercely defended their territory of mountain canyons. Like many indigenous tribes in California, the Yana and especially the Yahi suffered heavy population losses when European settlers entered their territory during the California Gold Rush of 1848–55; prior to this the Yahi probably numbered several hundred, while the total Yana in the larger region numbered around 3,000.

In 1865, the Yahi were attacked in the Three Knolls Massacre, in which 40 of them were killed. Although 33 Yahi survived to escape, cattlemen killed about half of the survivors. The last survivors, including Ishi and his family, went into hiding for the next 44 years. Their tribe was popularly believed to be exterminated.

The gold rush brought tens of thousands of miners and settlers to northern California, putting pressure on native peoples. Gold mining poisoned water supplies and killed fish; deer became scarcer. The settlers brought new infectious diseases such as smallpox and measles. The northern Yana group was wiped out while the central and southern groups (who later became part of Redding Rancheria) and Yahi suffered drastic losses. Searching for food, they came into conflict with settlers, who set bounties of 50 cents per scalp and 5 dollars per head on the natives. In 1865, settlers attacked a group of Yahi while they were asleep.

Richard Burrill wrote, in Ishi Rediscovered:

In 1865, near the Yahi's special place, Black Rock, the waters of Mill Creek turned red at the Three Knolls Massacre. "Sixteen" or "seventeen" Indian fighters killed about forty Yahi, as part of a retaliatory attack for two white women and a man killed at the Workman's household on Lower Concow Creek near Oroville. Eleven of the Indian fighters that day were Robert A. Anderson, Harmon (Hi) Good, Sim Moak, Hardy Thomasson, Jack Houser, Henry Curtis, his brother Frank Curtis, as well as Tom Gore, Bill Matthews, and William Merithew. W. J. Seagraves visited the site, too, but some time after the battle had been fought.