Sigurd Frosterus (4 June 1876 – 2 March 1956) was a Finnish architect, art critic and art collector.

Born in Asikkala, Frosterus graduated from Helsinki University with a degree in art history in 1899, and earned a diploma of architecture in 1902 at Polytechnical Institute.^[1] In 1920 he earned his doctorate at the University with a dissertation on the use of colour in art.^[2]

Frosterus had an architectural office in 1902–1904 with Gustaf Strengell and in 1918–1935 with Ole Gripenberg.^[2] Frosterus and Strengell designed villas and manor houses, including Tamminiemi, which was an official residence of the President of Finland from 1940 until 1981.^[1]

Frosterus's best-known work is the Stockmann department store in Helsinki. The architecture competition was held in 1916, but the building was not finished until 1930.^[2]

Frosterus inspired numerous Finnish artists with his art theories and criticism. He collected post-impressionist art.^[3]

Frosterus was the editor of Arkkitehti magazine in 1908–1911, and published books on art theory. He was also a talented watercolour painter.^[2]

• Inkeroinen power plant in Kymi 1923
• Vanajanlinna Manor, 1924
• Stockmann, Helsinki centre 1930
• Isohaara power plant in Kemijoki 1949
• Helsingin säästöpankki, bank head office, 1932
• Yhdyspankki, bank head office, 1936