Pittsburg is the northernmost New Hampshire municipality. It shares an international border with
Québec province,
Canadato its west and north, and borders the states of
Maine (to the east) and
Vermont (a very small portion to the southwest). Directly to the south is
Clarksville. Pittsburg is the only New Hampshire municipality to border Canada, the only one that borders both Maine and Vermont, and the only one to share a land border with Vermont. Pittsburg contains the only part of New Hampshire west of the
Connecticut River, as that river defines the Vermont state line from Clarksville southward. New Hampshire's only Canadian border crossing is located at the northern end of town at the terminus of
U.S. Route 3. The western edge of Pittsburg is defined by
Halls Stream, being the "northwesternmost headwaters of the
Connecticut River", which defined (ambiguously) the border in the
Treaty of Paris of 1783.