John Langdon was an American politician and Founding Father from New Hampshire. He served as a delegate to the Constitutional Convention, signed the United States Constitution, and was one of the first two United States senators from New Hampshire.

The Constitutional Convention took place in Philadelphia from May 25 to September 17, 1787. Although the convention was intended to revise the league of states and first system of government under the Articles of Confederation, the intention from the outset of many of its proponents, chief among them James Madison of Virginia and Alexander Hamilton of New York, was to create a new frame of government rather than fix the existing one. The delegates elected George Washington of Virginia, former commanding general of the Continental Army in the late American Revolutionary War (1775–1783) and proponent of a stronger national government, to become President of the convention. The result of the convention was the creation of the Constitution of the United States, placing the Convention among the most significant events in American history.