Kacperski tenement house in Warsaw, Historical tenement house in Śródmieście, Poland.

The Kacperski tenement house stands at the corner of Marszałkowska and Polna streets by Unii Lubelskiej Square, a nine-storey building erected in 1911–1913 for the merchant Stanisław Kowalski. Its trapezoidal plan earned it the nickname "the iron" and comparisons to New York's Flatiron Building. A narrow well courtyard in the centre brings light to the stairwells and corridors.

Built in 1911–1913 for Stanisław Kowalski, a provincial merchant who invested his capital in the property; the architect's name is unknown. The nine-storey building was one of Warsaw's two tallest residential houses when it opened in 1914. During the war it housed a cell of the Home Army's intelligence bureau; the roof was destroyed and two floors damaged, but the building survived in good condition. Renovated in 1955–1956, it lost part of its decoration.

The building's name comes from Stanisław and Kazimiera Kacperski, its long-standing owners, who sold it shortly before World War II for 1.3 million złoty to the State Pension Institution. During the war the site housed a cell of the Studies Bureau of the Second Department of the Home Army. In 1933 the house appeared in the feature film Dwanaście krzeseł, and part of the 2012 music video Widok by Marika and Spokoarmia was shot there.

The building can be viewed from the street to see its facade details and architectural features. It remains accessible for exterior observation in its current state after post-war reconstruction work.

The building's nickname "żelazko" (the iron) comes from its trapezoidal plan, which drew comparisons with New York's Flatiron Building. Its narrow central light well, into which the staircase and corridor windows open, was later covered at second-floor level with glass bricks in 1970.