The Saar rises close to the watershed of its basin and is the "outermost" tributary of the Werra, and thus of the Weser.

Its source lies near the Dreistromstein in the Thuringian Forest, a marker indicating the confluence of the three basins of the Rhine, the Weser, and the Elbe.

[2] It originates at an elevation of 800 metressouthwest of Siegmundsburg on the northwest slope of Dürre Fichte (861 m) in the Thuringian Slate Mountains.

The Saar flows west then southwest through the Saargrund valley, passing the settlement of the same name.

In its final kilometre the stream leaves the forest and reaches the Schirnrod, before merging with the Werra in Schwarzenbrunn, both settlements belonging to the town of Eisfeld.