William Emerson "Bill" Brock III (born November 23, 1930) was a Republican United States Senator from Tennessee from 1971 to 1977. He was the grandson of William Emerson Brock (I), who was a Democratic Senator from Tennessee from 1929 to 1931.

Brock was a native of Chattanooga, Tennessee, where his family owned a well-known candy company. He graduated from Washington and Lee College in Lexington, Virginia in 1953 and subsequently served in the United States Navy until 1956. He worked with his family's business until being elected to the United States House of Representatives from the Third District of Tennessee, which included Chattanooga, in 1962, as a Republican.

Brock served four terms in the House and then won the Republican nomination to face three-term incumbent U.S. Senator Albert Gore, Sr. in 1970, defeating country singer Tex Ritter in the primary. Brock's campaign was able successfully to make an issue of Gore's friendship with the Kennedy family and Gore's voting record, which was somewhat liberal by Southern standards, and defeated him.

While in the Senate, Brock was a darling of the conservative movement but was less than overwhelmingly popular at home; his personality was somewhat distant by the standards of most politicians. He was sensed as being vunerable in the 1976 election by Democrats and several prominent Democrats ran in the 1976 Democratic Senate primary for the right to challenge him. The most prominent and best-known name, at least initially, was probably 1970 gubernatorial nominee John Jay Hooker; somewhat surprisingly to most observers, he would be defeated by Jim Sasser. Sasser was able to exploit both lingering resentment of the Watergate affair, which had concluded only about two years earlier, but his most effective campaign strategy was to emphaisize how the affluent Brock, through skillful use of the tax code by his accountants, had been able to pay less than $2000 in income tax the previous year, an amount considerably less than that paid by many Tennesseans of far more modest means.

Brock went on to lose the election. However, he did not become unemployed. Upon the close of his Senate term, he became the new chairman of the Republican National Committee, a position he held from 1977 to 1981. Upon the election of Ronald Reagan as President, Brock was appointed U.S. Trade Representative, a position he maintained until 1985 when he was made Secretary of Labor.

Brock left government in 1987 and became a consultant in the Washington, D.C. area. By this point, he had become a legal resident of Maryland. In 1994, he won the Republican senatorial nomination to face Democratic senator Paul Sarbanes, who defeated him handily. Brock is currently a resident of Annapolis, Maryland.