Austrian diplomat and administrator who served as president of the German federal diet at Frankfurt in 1850, where he repeatedly clashed with Prussia's representative Otto von Bismarck (Bismarck, Otto von).

After the suppression of the 1848–49 revolutions in Germany and Austria, Thun, an astute politician, headed the Austrian delegation to the reestablished German diet at Frankfurt. As president of that body and spokesman for the conservative Austrian faction, he opposed Prussia's (Prussia) proposals—forcefully presented by Bismarck—for a more tightly centralized German confederation led by Prussia. Thun served as minister to Prussia (1852–54) and as minister to Russia (1859–63). After retiring from the diplomatic service he was active in Bohemian and Austrian internal politics.