John Underhill (c. 1608/09 – 21 July 1672) was an early English settler and soldier in the Massachusetts Bay Colony, the Province of New Hampshire, where he also served as governor; the New Haven Colony, New Netherland, and later the Province of New York, settling on Long Island. Hired to train militia in New England, he is most noted for leading colonial militia in the Pequot War (1636–1637) and Kieft's War which the colonists mounted against two different groups of Native Americans. He also published an account of the Pequot War.

John Underhill was one of three children of John Underhill (1574–1608) and Leonora Honor Pawley. His great-grandfather Sir Hugh Underhill was Keeper of the Wardrobe for Queen Elizabeth at Greenwich, and his grandfather Thomas Underhill held the same position at Kenilworth Castle for Elizabeth's favorite, Robert Dudley, 1st Earl of Leicester.

John Underhill was born in ca 1608/09 in the Netherlands. His family had escaped to the Netherlands after a failed plot by the Earl of Essex to overthrow the Queen. There they stayed in Bergen op Zoom, a heavily fortified city. John Underhill was sergeant in the company of Captain Roger Orme. He died there in October 1608 and is buried in the Church of St. Gertrude.

Following his father's death, John Underhill and his siblings lived with their mother and a group of Puritan exiles in the Netherlands. While there he received military training as a cadet in the service of Philip William, the Prince of Orange. He also married a Dutch girl, Helena (Heylken) de Hooch on 12 December 1628 in the Kloosterkerk, The Hague, Holland. They had one child born in the Netherlands before emigrating, Deborah Underhill, and two other children after emigrating: Elizabeth (born 1635) and John Underhill (1642–1692).

In 1630, Underhill was hired by the Massachusetts Bay Colony with the rank of captain and asked to help train the colony's militia. He and his Dutch wife emigrated to the Massachusetts Bay Colony on the Arbella that year. In May 1634 he was appointed to the General Court, and in July was elected a selectman for Boston. He started the first construction of the fortification on Castle Island at Boston.

Early in 1636 he was sent to Salem to arrest Roger Williams, considered a heretic by the Puritans. But, Williams had already fled to Rhode Island. In August 1636 Underhill led an expedition to Block Island.

Using the Narragansett killing of a Plymouth Colony-exile and the mistaken killing of an English pirate by the Pequot people as a pretext, the English colonists decided to go to war against the Pequots who controlled the regional wampum trade and coastal lands desired by the Puritans. In September 1637 Underhill headed the militia as it undertook the Pequot War. They first went to the fort at Saybrook, in present-day Connecticut. Joining with Mohegan allies and Connecticut militia under Captain John Mason, they attacked the fortified Pequot village near modern Mystic. They set fire to the village, killing any who attempted to flee. About 400 Pequot died in what came to be called the Mystic massacre.

Underhill led other expeditions that joined in hunting down the surviving Pequot. He published an account of his service as Newes from America; Or, A New and Experimentall Discoverie of New England; Containing, A True Relation of Their War-like Proceedings These Two Yeares Last Past, with a Figure of the Indian Fort, or Palizado (London, 1638).