Eisaku Satō (佐藤 栄作, Satō Eisaku; 27 March 1901 – 3 June 1975) was a Japanese politician who served as prime minister of Japan from 1964 to 1972. He was the third longest-serving and the second longest nonconsecutive serving Japanese prime minister. Satō is best remembered for securing the return of Okinawa in 1972, and for winning the Nobel Peace Prize in 1974, which stirred controversy. He was a former elite bureaucrat like his elder brother Nobusuke Kishi and a member of the Yoshida school like Hayato Ikeda.

Born in Yamaguchi Prefecture, Satō was a member of the Satō–Kishi–Abe family and the younger brother of prime minister Nobusuke Kishi. Satō graduated from Tokyo Imperial University in 1924 and joined the Ministry of Railways. After the war, he entered the National Diet in 1949 as a member of the Liberal Party, and served in a series of cabinet positions under Shigeru Yoshida, including posts and telecommunications minister from 1951 to 1952, construction minister from 1952 to 1953, and chief cabinet secretary from 1953 to 1954. Satō later joined the Liberal Democratic Party and became finance minister from 1958 to 1960 under Nobusuke Kishi and international trade and industry minister from 1961 to 1962 under Hayato Ikeda.

In 1964, Satō succeeded Ikeda as LDP president and prime minister. He had the support of Japanese business and finance, and presided over a period of rapid economic growth. In foreign policy, he oversaw the normalization of diplomatic relations with South Korea and maintained close relations with the United States, allowing the U.S.–Japan Security Treaty to renew in 1970 and arranging for the return of Okinawa to Japan in 1972. In 1967, Satō introduced the "Three Non-Nuclear Principles" (the non-production, non-possession, and non-introduction of nuclear weapons), and in 1968 signed the Nuclear Non-Proliferation Treaty, for which he won the Nobel Peace Prize. However, it was later revealed that Satō had made secret agreements with the U.S. to allow violations of the principles. Facing mounting economic problems and falling approval ratings in the early 1970s, Satō resigned in 1972 and was succeeded as prime minister by Kakuei Tanaka, quickly losing his political influence when his protégé Takeo Fukuda did not succeed him.

Satō was born on 27 March 1901, in Tabuse, Yamaguchi Prefecture, the third son of businessman Hidesuke Satō and his wife Moyo. His father had worked in the Yamaguchi Prefectural Office, but quit in 1898, and started a sake brewing business in Kishida, Tabuse. The family had a history in sake brewing and had held the right for sake brewing for generations. Satō's great-grandfather was a samurai of the Chōshū Domain, with their outsized influence in Meiji era Japan, with more Meiji and Taisho prime ministers coming from Yamaguchi than any other prefecture. His two older brothers were Ichirō Satō, who would become a vice admiral, and Nobusuke Kishi, who served as prime minister from 1957 to 1960.

Satō studied German law at Tokyo Imperial University and in 1923, passed the senior civil service examinations. Upon graduation the following year, he became a civil servant in the Ministry of Railways. He served as Director of the Osaka Railways Bureau from 1944 to 1946 and Vice-Minister for Transport from 1947 to 1948.

Satō entered the Diet in 1949 as a member of the Liberal Party.

He served as Minister of Postal Services and Telecommunications from July 1951 to July 1952. Sato gradually rose through the ranks of Japanese politics, becoming chief cabinet secretary to then prime minister Shigeru Yoshida from January 1953 to July 1954. He later served as minister of construction from October 1952 to February 1953.

After the Liberal Party merged with the Japan Democratic Party to form the Liberal Democratic Party, Satō served as chairman of the party executive council from December 1957 to June 1958, followed by a post as minister of finance in the cabinet of his brother Nobusuke Kishi from 1958 to 1960. As minister of finance, Sato requested the US to fund conservatives.