Frank Forrester Church III (July 25, 1924 – April 7, 1984) was an American politician and lawyer. A Democrat, from 1957 to 1981 he served as a U.S. Senator from Idaho, and is currently the last Democrat to do so. He was the longest serving Democratic senator from the state and the only Democrat from the state who served more than two terms in the Senate. He was a prominent figure in American foreign policy and established a reputation as a member of the party's liberal wing.

Born and raised in Boise, Idaho, he enrolled at Stanford University in 1942 but left to enlist in the Army, where he served as a military intelligence officer in the China Burma India Theater of World War II. Following the end of the war, he completed his law degree from Stanford Law School and returned to Boise to practice law. Church became an active Democrat in Idaho and ran unsuccessfully for a seat in the state legislature in 1952. In 1956, he was elected to the United States Senate, defeating former Senator Glen Taylor in a closely contested primary election and incumbent Herman Welker in the general election.

As a senator, he was a protégé of then-Senate majority leader Lyndon B. Johnson, and was appointed to the Senate Foreign Relations Committee. In 1960, Church received national exposure when he gave the keynote speech at the 1960 Democratic National Convention. Considered a strong progressive and environmental legislator, he played a major role in the creation of a system of protected wilderness areas. Church was highly critical of the Vietnam War, despite initially supporting it; he co-authored the Cooper–Church Amendment of 1970 and the Case–Church Amendment of 1973, which sought to curtail the war. In 1975, he chaired the Senate Select Committee to Study Governmental Operations with Respect to Intelligence Activities, better known as the Church Committee, laying the groundwork for the Foreign Intelligence Surveillance Act of 1978.

Church belatedly sought the 1976 Democratic nomination for president, and announced his candidacy on March 18, 1976. Although he won primaries in Nebraska, Idaho, Oregon, and Montana, he withdrew in favor of former Georgia governor Jimmy Carter. Church was re-elected continuously to the Senate, defeating his Republican opponents in 1962, 1968, and 1974, until his defeat during the Republican wave of 1980. Following the end of his term, he practiced international law in Washington, D.C., specializing in Asian issues. Church was hospitalized for a pancreatic tumor on January 12, 1984, and he died less than three months later at his home in Bethesda, Maryland, on April 7, 1984.