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The Twelve-Mile Circle is an arc that makes up most of the boundary between the Commonwealth of Pennsylvania and the State of Delaware in the United States.

It is a circle with a twelve-mile radius, with the center of the circle in the center of the town of New Castle, Delaware. In 1750, the center of the circle was fixed at the cupola of the courthouse in New Castle. The Twelve-Mile Circle continues into the Delaware River. A small portion of the circle, known as the "Arc Line," also forms part of the Mason-Dixon line, separating Delaware and Maryland. The Twelve-Mile Circle is the only territorial boundary in the United States that is a true arc (excluding all boundaries defined by latitude and longitude, like much of the border between the US and Canada, which—when viewed with the North Pole as the center—are all also arcs).