Katri (Kaija) Anna-Maija Helena Siren (née Tuominen; October 23, 1920 in Kotka – January 15, 2001) was a Finnish architect married to another Finnish architect, Heikki Siren.
Kaija Siren graduated as an architect from the Helsinki University of Technology in 1948. She and her husband Heikki Siren set up their own architectural office in 1949. The Sirens worked together as architects their entire life. The Otaniemi Chapel is noted for its delicate balance between features of Finnish rural architecture and a modernism, influenced by Alvar Aalto's redbrick period of the 1950s. Their later work is noted for its monumentality.
She is buried in the Hietaniemi Cemetery in Helsinki.1954 Finnish National Theatre Small Stage, Helsinki, Finland
1956 Otaniemi Chapel, Espoo, Finland
1961 Orivesi Church, Orivesi, Finland
1965 Kallio Municipal Offices, Helsinki, Finland
1968 Ympyrätalo, Helsinki, Finland
1970 Lauttasaari School, Helsinki, Finland
1973 Brucknerhaus, Linz, Austria
1982 Graniittitalo, Helsinki, Finland
1983 The Conference Palace in Baghdad, Iraq