It forms an intersection of Polna, Marszałkowska, Bagatela, Puławska, Klonowa, and Boya-Żeleńskiego Streets, and Szucha Avenue.

The name of the square refers to the Union of Lublin, an agreement signed in 1569, which led to the creation of the Polish–Lithuanian Commonwealth.

The same year, it was surrounded to the south and west by the Lubomirski Ramparts, a line of fortifications erected around the city.

[12] In 1936, at 1 Polna Street near the square, the headquarters building of the Warsaw Fire Guard and its 3rd Division was opened.

[14][15] In 1938, an office skyscraper designed by Bohdan Pniewski was planned to be constructed next to the square, as the headquarters of Polish Radio.

[19] On 1 August 1944, following the outbreak of the Warsaw Uprising, the square was captured by the Polish resistance fighters from the Jeleń Division.

[21] In October 1948, in the Bromski Tenement, at 14 Bagatela Street, the first book and newspaper store of Klub Międzynarodowej Prasy i Książki was opened.

[22][23] The company would eventually form Empik, one of the largest commercial chains in Poland, selling books, international press and media products.

[26] In 1964, the roads at the square were renovated and remodeled, with the street layout being changed, and the tram tracks relocated to go through its middle rather than around it.

[30] In its place, in 2013, the Plac Unii skyscraper, that serves as an office building and a shopping centre, was opened.

[3][4] The square is surrounded mostly by six- and eight-storey-tall tenements, including historical buildings dating from the early 20th century.

[5] Next to the square, at 2 Puławska Street, is the Plac Unii skyscraper, which serves as an office building and a shopping centre.