The river Saalach has its spring at the south-eastern slopes of the Staffkogel, a 2115 m high peak of the Kitzbühler Alpen in Austria. As a small creek it runs to the east through the ski-resort of Saalbach-Hinterklemm until it bends north at Maishofen. It follows the broad valley to Saalfelden and, meanders furtheron through the narrow valley between the Leonganger and Loferer Steinberge and the Steinernes Meer to Lofer in north-western direction. There it enters a narrow gorge, famous for its white water rafting. Crossing the border to Bavaria (Germany) at Mellek it flows along the northern slopes of the Reiteralm known for its famous climbing routes. A short distance in front of Bad Reichenhall , a dam of a hydro-electrical power plant collects the waters of the Saalachsee. The power plant provides the energy for the railway line Salzburg, Freilassing, Bad Reichenhall to Berchtesgaden. Having passed Staufenegg, the Saalach leaves the mountains and enters the flat forests called Saalachau. Form Piding on north-eastwards the border to Austria follows the river for some 10 km to Freilassing at which northern community border it merges with the river Salzach.

The name has its origin from Saal, an old form of Salz (salt), and Ach which denotes a small river.