politician who figured prominently in solving the problems of the Norwegian-Swedish Union. He was the cowinner (with Fredrik Bajer) of the Nobel Prize for Peace in 1908.

Arnoldson became a railway clerk and rose to stationmaster (1871–81) but then left the railway to devote himself entirely to politics. In 1881 he was elected to the Riksdag, the Swedish parliament. A passionately devoted pacifist, Arnoldson supported the neutrality of the Nordic countries and in 1883 helped to found the Swedish Association for Peace and Conciliation.

From 1890, when the conflict between Norway and Sweden was critical, Arnoldson made use of all his powers, including his gift for inspiring oratory, in an attempt to shape public opinion in both countries in favour of a peaceful settlement. In 1905 he saw the result of his work in the mutually agreed dissolution of the union.