Atos Kasimir Wirtanen (27 January 1906 in Saltvik – 10 March 1979) was a Finnish left-wing intellectual, journalist, member of Finnish parliament (1936–1953) and cultural critic. He was born in Saltvik, Åland.
Wirtanen rose to parliament from the Social Democratic Party of Finland (SDP) list, but in 1946 he defected to the Finnish People's Democratic League (SKDL). Wirtanen was also active in the Socialist Unity Party (SYP) and served as its chairman from 1948 to 1955. In 1955 he led the SYP out of the SKDL. Wirtanen was fired twice (1941 and 1947) from the publication Arbetarbladet and once (1953) from the magazine Ny Tid. During the Continuation War, he was among the signatories of the "Petition of the Thirty-Three", which members of the peace opposition presented to President Risto Ryti.
Wirtanen was a close friend of Tove Jansson and a model for the Snufkin character, whose green hat is borrowed from Wirtanen. The first Moomin comic strips were published in October 1947 in Ny Tid. The series had to stop earlier than planned, as readers criticized it and especially the Moominpappa character for his bourgeois sympathies.
Wirtanen married dance artist Irja Hagfors in 1954. He died in 1979 in Helsinki.