Carl Christian August Michalsen was a Norwegian architect.
In Sandefjord, he designed the city's new church and the Nordstjernen Pharmacy.
From 1904 to 1909, Michalsen worked in Ålesund, largely in partnership with Alfred Christian Dahl under the firm name Michalsen & Dahl. This resulted in over twenty buildings in the Jugendstil style.
Afterwards, Michalsen returned to Cristiania (now Oslo) and was responsible for the expansion of the Oslo Stock Exchange before working in partnership with Ove Ekman and Einar Smith under the firm name Ekman, Smith & Michalsen from 1912. This firm erected numerous villas in Bygdøy and other areas in what was then Aker municipality, including the two grand villas in Bygdøynesveien 13 and 15. The buildings could be in Jugendstil or national romantic variants of neo-baroque and neoclassicism. After Ekman's death in 1921, Michalsen's son Eystein became a partner in the firm, which continued as Michalsen, Smith, Michalsen. This incarnation of the firm designed, among others, the building of Den norske Creditbank in Kirkegata 21 and Kongens gate 18 (1923–26) together with Kristian Biong, which received the Houen Foundation's Diploma for the building.
From the mid-1920s, father and son Michalsen worked together as Michalsen & Michalsen, operating stylistically on the border between neoclassicism and functionalism. From this period, examples include the Astoria Hotel in Oslo and the Student Society in Trondheim, both of which received the Houen Foundation's Diploma.
Michalsen served as chairman of the Norwegian Architects' Association in 1915–16.