William Floyd Weld (born July 31, 1945) is an American attorney, businessman, author, and politician who served as the 68th Governor of Massachusetts from 1991 to 1997. A Harvard and Oxford graduate, Weld began his career as legal counsel to the United States House Committee on the Judiciary before becoming the United States Attorney for the District of Massachusetts and later, the United States Assistant Attorney General for the Criminal Division. He worked on a series of high-profile public corruption cases and later resigned in protest of an ethics scandal and associated investigations into Attorney General Edwin Meese.

Weld was elected Governor of Massachusetts in 1990. In the 1994 election, he was reelected by the largest margin of victory in Massachusetts history. In 1996, he was the Republican nominee for the United States Senate in Massachusetts, losing to Democratic incumbent John Kerry. Weld resigned as governor in 1997 to focus on his nomination by President Bill Clinton to serve as United States Ambassador to Mexico; due to opposition by socially conservative Senate Foreign Relations committee Chairman Jesse Helms, he was denied a hearing before the Foreign Relations committee and withdrew his nomination. After moving to New York in 2000, Weld sought the Republican nomination for Governor of New York in the 2006 election; when the Republican Party instead endorsed John Faso, Weld withdrew from the race.

Weld became involved in presidential politics in later years. In 2016, he left the Republican Party to become the Libertarian Party running mate of former Governor of New Mexico Gary Johnson. They received nearly 4.5 million votes, the highest number for a Libertarian ticket, and the best for any third-party ticket since 1996 with Ross Perot's Reform Party.

Returning to the Republican Party, Weld announced in April 2019 that he would challenge President Donald Trump in the 2020 Republican primaries, launching his campaign. He won his first and only delegate of the primaries in the Iowa caucus in February, making him the first Republican since Pat Buchanan in 1992 to win a delegate while running against an incumbent president. Weld suspended his campaign on March 18, 2020, shortly after Trump's delegate count made him the presumptive Republican nominee, and ultimately placed second in 22 states and second overall with 2.4% of the popular vote, collecting relevant percentages of up to 13% in protest-votes against Trump in several states. He endorsed Democrat Joe Biden seven months later.