The King-Byng Affair refers to a 1926 Canadian constitutional crisis that occurred when the Governor General of Canada, Lord Byng of Vimy refused a request by the Prime Minister of Canada, William Lyon Mackenzie King, to dissolve parliament and call a general election.

The crisis was watched closely by both the Canadian and British governments, and came to redefine the role of Governor General not only in Canada but throughout the dominions. It was also a major impetus in negotiations at Imperial Conferences held in the late 1920s that led to the Statute of Westminster 1931.

According to British Empire constitutional theory a governor general represented both the Sovereign and the British government, a situation that had evolved with Lord Byng's predecessors and the Canadian government as well as the Canadian people into a tradition of non-interference in Canadian political affairs.

In September 1925, Prime Minister Mackenzie King requested a dissolution of Parliament to call an election, which Lord Byng granted. In the 1925 election, the Conservative Party won the most seats, but not a majority. Counting on the support of the Progressive Party to overcome the Conservative minority, Mackenzie King, the Liberal Party leader, did not resign as Prime Minister and remained in power with a minority government.

Then, a political scandal in the Ministry of Customs and Excise became public, and in Parliament the Conservative Party alleged that the corruption extended to the highest levels of government, including the Prime Minister. Mackenzie King fired the Minister of Customs and promptly named him to the Senate, creating even more dismay among the members of the Progressive Party, who had already been withdrawing their support for the Liberal government.

Facing a third vote on the question of government corruption, and having already lost two previous votes on questions of procedure, Mr. King went to the Governor General seeking a dissolution of Parliament. Lord Byng used his reserve power to refuse the request thus igniting the crisis. Prime Minister King requested that before any decision was made, Lord Byng consult the British government which he represented. Governor General Byng again refused, citing non-interference in Canadian affairs.

The next day, King presented Lord Byng with an Order-in-Council seeking the dissolution of Parliament, which Lord Byng refused to sign. King resigned and, as a result, Canada was left temporarily without a Prime Minister and government until the Governor General invited Arthur Meighen to form a government. Meighen did so, but within a week lost a non-confidence vote in the House of Commons. Prime Minister Meighen requested a dissolution of Parliament, which was granted by Byng, and an election was called.

Politically, much was made of the 'Byng-King Crisis' during the election campaign. The Liberals were returned to power with a clear majority and King as Prime Minister. Once in power, King's government sought to redefine the role of Governor General as a representative of the sovereign and not the British government at a Commonwealth conference. The change was agreed, replacing the concept of a unitary crown and a governor general's role as an agent of the British government with the concept of a multiple crown worn by a shared monarch, the monarch (now King of Canada) being the only link, in the person of his governor general, binding the sovereign realms of the Empire. From 1927 onwards, under the Royal and Parliamentary Titles Act, Commonwealth governors general ceased to be the agents of the Imperial or British government in each dominion — this role was to be assumed by a British High Commissioner, whose duties were soon recognized to be virtually identical to those of an ambassador.

On leaving Canada on September 30, 1926, Lord Byng returned to England. Despite the political crisis, he left a much-respected man.

The King-Byng Affair was the most controversial use of a Governor General's powers until the Australian constitutional crisis of 1975 in which the Australian Governor General dismissed Prime Minister Gough Whitlam.

The King-Byng Affair is occasionally called the "King Byng Thing" by Canadian political scientists and historians in casual conversation or informal writting. Most notably by author Will Ferguson who writes about Canadian history, politics, and culture with a comedic slant.