Adlai Ewing Stevenson II (/ˈædleɪ/; February 5, 1900 – July 14, 1965) was an American politician and diplomat who was the United States ambassador to the United Nations from 1961 until his death in 1965. He previously served as the 31st governor of Illinois from 1949 to 1953 and was the Democratic Party nominee for president of the United States in 1952 and 1956, losing both elections to Dwight D. Eisenhower.

Stevenson was the grandson of Adlai Stevenson, the 23rd vice president of the United States. He was raised in Bloomington, Illinois. After a short stint in the U.S. Navy at the end of World War I, he became a lawyer. He served in many positions in the federal government during the Franklin D. Roosevelt and Harry Truman presidential administrations, including in the Department of the Navy, and the State Department during World War II. In 1945, he served on the committee that created the United Nations, and was a member of the initial U.S. delegations to the UN.

In 1948, Stevenson was elected governor of Illinois, defeating incumbent governor Dwight H. Green in an upset. As governor, Stevenson reformed the state police, cracked down on illegal gambling, improved the state highways, and attempted to cleanse the state government of corruption. Stevenson also sought, with mixed success, to reform the Illinois state constitution and introduced several crime bills in the state legislature.

In 1952 and 1956, Stevenson was chosen as the Democratic nominee for president but was defeated in a landslide by Eisenhower both times. In 1960, he unsuccessfully sought the Democratic presidential nomination for a third time at the Democratic National Convention. After President John F. Kennedy was elected, he appointed Stevenson as the United States ambassador to the United Nations (UN). Two major events Stevenson dealt with during his time as UN ambassador were the Bay of Pigs Invasion of Cuba in April 1961 and the Cuban Missile Crisis in October 1962. He was still serving as UN ambassador through the Lyndon Johnson administration when he suffered a heart attack during a visit to London on July 14, 1965, dying later that day at the age of 65. He is buried in Evergreen Cemetery in his hometown of Bloomington, Illinois.

Adlai Ewing Stevenson II was born in Los Angeles, California, in a neighborhood that is now designated as the North University Park Historic District. His home and birthplace at 2639 Monmouth Avenue has been designated as a Los Angeles Historic-Cultural Monument. He was a member of a prominent Illinois political family. His grandfather and namesake Adlai Stevenson was Vice President of the United States under President Grover Cleveland from 1893 to 1897. His father, Lewis Stevenson, never held an elected office, but was appointed Illinois Secretary of State (1914–1917) and was considered a strong contender for the Democratic vice-presidential nomination in 1928. A maternal great-grandfather, Jesse W. Fell, had been a close friend and campaign manager for Abraham Lincoln in his 1858 US Senate race; Stevenson often referred to Fell as his favorite ancestor. Stevenson's eldest son, Adlai E. Stevenson III, became a U.S. Senator from Illinois (1970–1981). His mother was Helen Davis Stevenson, and he had an older sister, Elizabeth Stevenson Ives, an author who was called "Buffie". Actor McLean Stevenson was a second cousin once removed. He was the nephew by marriage of novelist Mary Borden, and she assisted in the writing of some of his political speeches.

Stevenson was raised in the city of Bloomington, Illinois, and his family was a member of Bloomington's upper class and lived in one of the city's well-to-do neighborhoods. On December 30, 1912, at the age of twelve, Stevenson accidentally killed Ruth Merwin, a 16-year-old friend, while demonstrating drill technique with a rifle, inadvertently left loaded, during a party at the Stevenson home. Stevenson was devastated by the accident and rarely mentioned or discussed it as an adult, even with his wife and children. However, in 1955 Stevenson heard about a woman whose son had experienced a similar tragedy. He wrote to her that she should tell her son that "he must now live for two", which Stevenson's friends took to be a reference to the shooting incident.

Stevenson left Bloomington High School after his junior year and attended University High School in Normal, Illinois, Bloomington's "twin city", just to the north. He then went to boarding school in Connecticut at The Choate School (now Choate Rosemary Hall), where he played on the tennis team, acted in plays, and was elected editor-in-chief of The Choate News, the school newspaper. Upon his graduation from Choate in 1918, he enlisted in the United States Naval Reserve for World War I. He served as a seaman apprentice while undergoing training but the Armistice of November 11, 1918 ended the war while he was still in training, and he was discharged as a seaman second class in January 1919.

He attended Princeton University, becoming managing editor of The Daily Princetonian, a member of the American Whig-Cliosophic Society, a member of the Quadrangle Club, and received a B.A. degree in 1922 in literature and history. Under prodding from his father, he then went to Harvard Law School, but found law to be "uninteresting", and withdrew after failing several classes. He returned to Bloomington where he wrote for the family newspaper, The Daily Pantagraph, which was founded by his maternal great-grandfather Jesse Fell. The Pantagraph, which had one of the largest circulations of any newspaper in Illinois outside the Chicago area, was a main source of the Stevenson family's wealth. Following his mother's death in 1935, Adlai inherited one-quarter of the Pantagraph's stock, providing him with a large, dependable source of income for the rest of his life.