Belvedere 21, formerly 21er Haus or Einundzwanziger Haus, is a modernist style steel and glass building designed by Austrian architect Karl Schwanzer (1918–1975). Originally constructed as the Austrian pavilion or temporary showroom for the Expo 58 in Brussels, it was later transferred to Vienna to house the Museum of the 20th Century, which explains why it was first nicknamed "20er Haus". Between 1979 and 2001, the building also acted as a depository for contemporary art works. From 2009 to 2011, it was remodeled by the architect Adolf Krischanitz and consequently renamed 21er Haus to reflect the 21st century. It was renamed Belvedere 21 in 2018.

Expo 58, also known as the 1958 Brussels World's Fair, was a world's fair held on the Heysel/Heizel Plateau in Brussels, Belgium, from 17 April to 19 October 1958. It was the first major world's fair registered under the Bureau International des Expositions (BIE) after World War II.