The five villages of the municipality are located in an elongated valley on the shores of Lake Grundl, which is framed on three sides by the foothills of the Tote Gebirge mountain range, which rise about 1000 meters. The most prominent mountains framing this valley are the Hundskogel (1748 m), the Backenstein (1772 m) and the Reichenstein (1913 m) in the north, the Elm (2128 m), the Große Hochkasten (2389 m) and the Weiße Wand (2198 m) in the east, the Türkenkogel (1756 m) and the Röthelstein (1614 m) in the south. The highest mountain of Grundlsee is the Große Hochkasten with 2389 m at the border to Upper Austria.
The Styrian Sea Grundlsee
is with 4.22 km² the largest lake in Styria and is drained by the Grundlseer Traun. It is fed by the Stimitz, the Zimitzbach and the Toplitz, the drainage stream of the Toplitzsee, which is located in the municipality. In addition, the Dreibrüdersee, the Elmsee, the Henarsee, the Kammersee, the Vordere and the Hintere Lahngangsee are located in the municipal area, as well as the source waters of the Traun, the so-called Traun-Ursprung. A large part of the Grundlsee municipal area is under strict nature protection.