His results in the triennial Candidates tournaments, held to determine the challenger to the world champion, showed a steady improvement: 5th at Zürich in 1953; equal 3rd at Amsterdam in 1956; 3rd in Yugoslavia in 1959; 1st at Curaçao in 1962. In 1963 he defeated Mikhail Botvinnik 12.5 - 9.5 to become world chess champion.
Petrosian defended his title in 1966, defeating Boris Spassky 12.5-11.5. In 1969 he was beaten by Spassky 12.5-10.5.