Carl Theodor Höijer (20 February 1843, Helsinki – 31 October 1910, Helsinki) was an Finnish architect. He designed a large number of buildings in central Helsinki. He was the first architect in Finland who managed to pursue a truly successful career without holding an official office. He has been described as the foremost architect working in Neo-Renaissance style in Finland.
Theodor Höijer was the son of a mirror manufacturer, Henrik Höijer. The father wanted Theodor Höijer to go into the family business and needed a co-worker with skills in architecture. Theodor Höijer never finished high school and had to acquire his skills outside the normal institutions. Initially he worked as an assistant to the county architect of Turku, Georg Theodor Chiewitz, and later studied at the Royal Institute of Art in . Here he was heavily influenced by his teacher Fredrik Wilhelm Scholander. He returned to Finland in 1868. In the meantime, his father's business had ceased to operate and he began working independently as an architect. Although he applied for a job at the government agency responsible for supervising building activity he was never accepted and worked his whole life as an independent, privately operating architect.
His first commissions came from a large industrial brewery, the Diocese of Helsinki and Helsinki Municipality. His breakthrough came in 1876, when he was responsible for designing the buildings of a large industrial exhibition. Around the same time he received a prestigious commission from the industrialist Fredrik Wilhelm Grönqvist to build a large tenement house in a central Helsinki (1880), and subsequently got more and more commissions. These were often from the upper middle class, and many times concerned large buildings in central Helsinki (such as Hotel Kämp, 1886). To a degree it can be said that central Helsinki is characterised by the buildings of Höijer. He has been described as the foremost architect working in Neo-Renaissance style in Finland.
The design for Ateneum (built 1885) marked the high point of Höijer's career. During the 1890s the building activity in Helsinki dropped, and when it increased again around 1900 his style was becoming out of fashion. He was at this time heavily criticised by the younger generation of architects in Finland. He continued to be active but on a smaller scale, with less prestigious commissions and together with a partner, the engineer Robert Huber.