Isaac Ingalls Stevens (March 25, 1818 – September 1, 1862) was an American military officer and politician who served as governor of the Territory of Washington from 1853 to 1857, and later as its delegate to the United States House of Representatives. During the American Civil War, he held several commands in the Union Army. He was killed at the Battle of Chantilly, while at the head of his men and carrying the fallen colors of one of his regiments against Confederate positions. According to one account, at the hour of his death Stevens was being considered by President Abraham Lincoln for appointment to command the Army of Virginia. He was posthumously advanced to the rank of Major General. Several schools, towns, counties, and lakes are named in his honor.

Descended from early American settlers in New England, Stevens – a man who stood just 5 ft 3 in (1.60 m) tall – overcame a troubled childhood and personal setbacks to graduate at the top of his class at West Point before embarking on a successful military career. He was a controversial and polarizing figure as governor of the Washington Territory, where he was both praised and condemned. He was described by one historian as the subject of more reflection and study than almost the rest of the territory's 19th-century history combined. Stevens' marathon diplomacy with Native American tribes sought to avoid military conflict in Washington; however, when the Yakama War broke out as Native Americans resisted European encroachment, he prosecuted it mercilessly. His decision to rule by martial law, jail judges who opposed him, and raise a de facto personal army led to his conviction for contempt of court, for which he famously pardoned himself, and a rebuke from the President of the United States. Nonetheless, his uncompromising decisiveness in the face of crisis was both applauded by his supporters and noted by historians.

Isaac Stevens was the father of Hazard Stevens, the hero of the Battle of Suffolk and one of the first men to summit Mount Rainier.

Stevens was born in North Andover, Massachusetts, to Isaac Stevens and Hannah Stevens (née Cummings), a descendant of early Puritan settlers from a gentry family that had produced several distinguished members of the clergy and military. He was a cousin of brothers Moses Tyler Stevens and Charles Abbot Stevens. As a young man, he was noted for his intelligence, particularly his mathematical acuity. His diminutive stature – in adulthood he stood 5 ft 3 in (1.60 m) tall – has been attributed to a possible congenital gland malfunction.

Stevens resented his father, described by historian Kent Richards as a "stern taskmaster", whose unrelenting demands on his son pushed the young man to his breaking point. While working on the family farm, Stevens once nearly died of sunstroke. After Stevens' mother died in a carriage accident, his widowed father married a woman whom Stevens disliked. According to Stevens, he came close to suffering a mental breakdown in his youth.

Stevens graduated from the male prep school Phillips Academy in 1833 and was accepted to the United States Military Academy at West Point. He graduated in 1839 at the top of his class.

He married Margaret Hazard Stevens (née Lyman Hazard), daughter of Benjamin Hazard, a local political figure, on September 8, 1841, whilst stationed at Fort Adams, near Newport, Rhode Island.

Stevens was the adjutant of the Corps of Engineers during the Mexican–American War, seeing action at the siege of Vera Cruz and at Cerro Gordo, Contreras, and Churubusco. In the latter fight, he caught the attention of his superiors, who rewarded him with the brevet rank of captain. He was again cited and breveted for gallantry at the Battle of Chapultepec, this time to the rank of major. Stevens participated in combat at Molino del Rey, and the Battle for Mexico City, where he was severely wounded. He later wrote a book on his adventures, Campaigns of the Rio Grande and Mexico, with Notices of the Recent Work of Major Ripley (New York, 1851).