Viktor Lvovich Korchnoi (Russian: Ви́ктор Льво́вич Корчно́й; IPA: [vʲiktər lʲvovʲɪtɕ kɐrtɕˈnoj]; 23 March 1931 – 6 June 2016) was a Soviet (until 1976) and Swiss (since 1994) chess grandmaster and writer. He is considered one of the strongest players never to have become World Chess Champion.
Born in Leningrad, Soviet Union, Korchnoi defected to the Netherlands in 1976, and later resided in Switzerland from 1978, becoming a Swiss citizen. Korchnoi played three matches against GM Anatoly Karpov. In 1974, he lost the Candidates final to Karpov, who was declared World Champion in 1975 when GM Bobby Fischer refused to defend his title. He then won two consecutive Candidates cycles to qualify for World Championship matches with Karpov in 1978 and 1981, losing both.
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 a four-time USSR chess champion, a five-time member of Soviet teams that won the European championship, and a six-time member of Soviet teams that won the Chess Olympiad. He is the only player to have won or drawn (in individual game(s)) against every World Chess Champion, disputed or undisputed, since the world chess championship interregnum of World War II. In September 2006, he won the World Senior Chess Championship.