The Webster–Ashburton Treaty, signed August 9, 1842, was a treaty that resolved several border issues between the United States and the British North American colonies (the region that became Canada). Signed under John Tyler's presidency, it resolved the Aroostook War, a nonviolent dispute over the location of the Maine–New Brunswick border. It:

Established the border between Lake Superior and the Lake of the Woods, originally defined in the Treaty of Paris in 1783;

Reaffirmed the location of the border (at the 49th parallel) in the westward frontier up to the Rocky Mountains defined in the Treaty of 1818;

Defined seven crimes subject to extradition;

Called for a final end to the slave trade on the high seas;

Agreed that the two parties would share use of the Great Lakes.The treaty retroactively confirmed the southern boundary of Quebec that land surveyors John Collins and Thomas Valentine had marked with stone monuments in 1771–1773. The treaty intended that the border be at 45 degrees north latitude, but the border is in some places nearly a half mile north of the parallel. The treaty was signed by United States Secretary of State Daniel Webster and British diplomat Alexander Baring, 1st Baron Ashburton.