The square was built in 1821 at the site of the ruins of a medieval Capuchin monastery, which had been abolished during the reign of Habsburg Emperor Joseph II. The square was used for ceremonial purposes during the Congress of Ljubljana, after which it was named. After the congress, a park was laid out in the center of the square, which soon acquired the name Star Park (Slovene: Park Zvezda, German: Sternallee) due to its layout. During the communist period it was renamed Revolution Square and few years later Liberation Square (Slovene: Trg osvoboditve), but the local population continued to use the old name. In 1990, it regained its original name.
The square has had a highly symbolic role in modern Slovenian history. On October 29, 1918, independence from Austrian-Hungarian rule and the establishment of the State of Slovenes, Croats and Serbs was proclaimed during a mass demonstration on the square. In May 1945, the Yugoslav Communist leader Josip Broz Tito first visited Slovenia after World War II and held a speech on the balcony of the University of Ljubljana, which faces the square.
On June 22, 1988, the first free mass demonstration was held on the square demanding the release of four Slovene journalists imprisoned by the Yugoslav army. The demonstration marked the beginning of the Slovenian spring, which culminated in the declaration of Slovenia’s independence on June 25, 1991. Several important buildings face the square. Among them, there is the early Baroque Ursuline Church of the Holy Trinity, the Kazina building, one of the few Neoclassical buildings remaining in Ljubljana after the earthquake of 1895, the Slovenian Philharmonic building, and the rectorate of the University of Ljubljana, formerly the seat of the Provincial Diet of the Duchy of Carniola. The prestigious Slovenska matica publishing house also has its seat on the square.
In the late 1930s, the square was renovated by the prominent Slovene architect Jože Plečnik. New trees on the park were planted, most of which are still there today. In 1954, after the formal annexation of Zone B of the Free Territory of Trieste to Yugoslavia, an anchor was placed in the park to symbolize victory over Italian expansionism and the union of the Slovenian Littoral with the rest of Slovenia.