Kosonen decided to return to Finland in 1905. He was involved in the activities of the Social Democratic Party, and served as editor of the "Kansan Lehti" newspaper. During the 1905 Russian revolution, Kosonen organised shelter for refugees from the Baltic region of the Russian Empire, also helped organise the December 1905 Tampere bolshevik conference. In February 1906, he helped Latvian revolutionary Jānis Čoke, who was later revealed to have taken part in the robbery of the Russian State Bank branch in Helsinki.