Thalweg (a German word compounded from Tal, valley, and Weg, way) is a term adopted into English usage for geography. It signifies the line of greatest slope along the bottom of a valley, i.e. a line drawn through the lowest points of a valley in its downward slope. It thus marks the natural direction of a watercourse.

The thalweg principle is the principle of determining national boundaries at the thalweg of a river separating two states. The precise drawing of river borders has been important on countless occasions; notable examples include the Shatt al-Arab between Iraq and Iran, and the Danube in central Europe.

A thalweg in geology is a line representing the profile of a land surface, drawn as a series of cross-sections that crosses all contour lines at right angles.

The word thalweg is also used in hydrology. It refers to a line drawn to join the lowest points along the entire length of a streambed or valley. This is occasionally called a valley line. The term is also sometimes used to refer to a subterranean stream that percolates under the surface and in the same general direction as the surface stream.