Viktor Lvovich Korchnoi (Russian: Виктор Львович Корчной, IPA: [vʲiktər ˈlʲvovʲɪtɕ kɐrtɕˈnoj]; 23 March 1931 – 6 June 2016) was a Soviet (before 1976) and Swiss (after 1980) chess grandmaster (GM) and chess writer. He is considered one of the strongest players never to have become World Chess Champion.

Born in Leningrad, Korchnoi defected to the Netherlands in 1976, and resided in Switzerland from 1978, becoming a Swiss citizen.

Korchnoi played four matches against GM Anatoly Karpov, three of which were official. In 1974, Korchnoi lost to Karpov in the Candidates Tournament final. After GM Bobby Fischer declined to defend his title against Karpov, Karpov was declared World Champion in 1975. In 1978 and 1981, Korchnoi won consecutive Candidates cycles and qualified to challenge Karpov for the World Chess Championship, but lost both matches. The two players also played a drawn training match of six games in 1971.

Korchnoi was a candidate for the World Championship on ten occasions (1962, 1968, 1971, 1974, 1977, 1980, 1983, 1985, 1988, and 1991). He was also four times a USSR Chess Champion, five times a member of Soviet teams that won the European championship, and six times a member of Soviet teams that won the Chess Olympiad. He played competitive chess until old age. At age 75, he won the 2006 World Senior Chess Championship and became the oldest person ever to be ranked among the world's top 100 players.

Korchnoi was born on 23 March 1931 in Leningrad, Russian SFSR, Soviet Union, to a Jewish mother and a Polish-Catholic father. His mother, Zelda Gershevna Azbel (1910—?), a daughter of the Yiddish writer Hersh Azbel, was a pianist and alumna of Leningrad Conservatory of Music; his father, Lev Merkuryevich Korchnoi (1910–1941), was an engineer, who worked at a candy factory.

Both parents came to Leningrad with their families from Ukraine in 1928: mother from Borispol and father from Melitopol. After their divorce, Victor lived with his mother until 1935, then with his father and paternal grandmother. They suffered under the siege of Leningrad during which Victor's father was killed in 1941. Victor's stepmother and adoptive mother Roza Abramovna Fridman then took responsibility for his upbringing. (She would later live with him in Switzerland.)

He graduated from Leningrad State University with a major in history.

He learned to play chess from his father at the age of five. In 1943, he joined the chess club of the Leningrad Pioneer Palace, and was trained by Abram Model, Andrei Batuyev, and Vladimir Zak. Model had earlier played a major role in the development of future World Champion Mikhail Botvinnik, while Zak, who later co-authored a book with Korchnoi, had also helped train future World Champion Boris Spassky.