Obersee, Lake section in Switzerland, Austria and Germany

The Obersee is the larger of the two parts of Lake Constance, covering 473 km² and stretching 63 km from Bregenz to Bodman-Ludwigshafen, with a maximum width of 14 km. Its main inflow is the Alpine Rhine, and it drains through the Seerhein at Konstanz into the Untersee. The lake borders Switzerland (Thurgau, St. Gallen), Austria (Vorarlberg), and Germany (Baden-Württemberg, Bavaria).

The Romans called the lake Lacus Venetus, Lacus Brigantinus and Lacus Constantinus. In the Middle Ages the name Lacus Bodamicus, after the village of Bodman, became dominant; it gradually came to cover the Untersee too, so the larger main lake received the separate name Obersee. The borders between the riparian states on the south-eastern main part have never been jointly fixed; only the Überlinger See lies entirely on German territory.

The name Obersee (Upper Lake) emerged in the Middle Ages, when Lacus Bodamicus, the name taken from the village of Bodman, gradually came to cover the Untersee as well, so a separate term was needed for the larger main lake. The Romans had called it Lacus Venetus, Lacus Brigantinus and Lacus Constantinus.

The Obersee stretches 63 km from Bregenz in the east to Bodman-Ludwigshafen in the west, with a maximum width of 14 km. Only the Überlinger See, the northwestern arm, lies entirely in German territory; the borders between Switzerland, Austria and Germany on the southeastern main part have never been jointly agreed. The southeastern shore meets the Appenzell Alps and the Allgäu Alps, and the Pfänder above Bregenz offers an outlook over the lake.

The southeastern end of the Obersee meets the Alps: the Appenzell Alps in Switzerland and the Allgäu Alps in Austria and Germany. Above Bregenz rises the Pfänder, a mountain with an outlook over the lake. The lake's borders between the riparian states in the southeastern main part were never jointly agreed; only the Überlinger See lies entirely on German territory.