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.

Frank Forrester Church III was born on July 25, 1924, in Boise, Idaho. He traced his ancestry from the East Coast of the United States, with his grandfather, Frank Forrester Church I, moving to Idaho during the height of the gold rush that followed the end of the Civil War. Church III was the younger of two sons of Frank Forrester Church II and Laura Bilderback Church. His older brother Richard Church became a career officer in the United States Marine Corps, and retired as a colonel.

His father co-owned a sporting goods store and took the sons on fishing, hunting, and hiking outings in the Idaho mountains. The family was reportedly very Catholic and conservative, with Church attending St. Joseph's School as a youngster, where he went by the nickname "Frosty." In his youth, Church admired William Borah, who represented Idaho in the United States Senate from 1907 until his death in 1940. When Borah died, Church walked by the open coffin in the rotunda of the State Capitol. He stated that "Because he was a senator, I wanted to become one, too." Church graduated from Boise High School in 1942, where he served as student body president. As a junior in 1941, he won the American Legion National Oratorical Contest, which resulted in him receiving sufficient funds to provide for his four-year enrollment at Stanford University in California, where he joined the Theta Xi fraternity.

Church left Stanford in 1942, at the age of 18, and enlisted in the Army following the attack on Pearl Harbor. He was called up the following year and attended officer candidate training at Fort Benning in Georgia. He trained at Camp Ritchie, as one of the Ritchie Boys, and was commissioned a lieutenant on his 20th birthday. In the army, he served as a military intelligence officer in the China Burma India Theater. He was inducted to the Infantry Hall of Fame at Fort Benning. Following the end of the war, he was discharged in 1946.

In June 1947 he married Bethine Clark, daughter of Chase Clark, a former Democratic governor of Idaho and the federal judge for the state. The wedding took place at the secluded Robinson Bar Ranch (44°14′49″N 114°40′41″W / 44.247°N 114.678°W / 44.247; -114.678), the Clark family's ranch in the mountains east of Stanley (and now owned by singer Carole King, since 1981). The two had a happy marriage and often showed their affection in public. He entered Harvard Law School that fall and after one year, Church transferred to Stanford Law School, when he thought the cold Massachusetts winter was the cause of a pain in his lower back. The pain did not go away and the problem was soon diagnosed as testicular cancer. After one of his testicles and glands in his lower abdomen were removed, Church was given only a few months to live. However, he rebounded from the illness after another doctor started X-ray treatments. This second chance led him to later reflect that "life itself is such a chancy proposition that the only way to live is by taking great chances." In 1950, Church graduated from Stanford Law School and returned to Boise to practice law and teach public speaking at Boise Junior College (now Boise State University).