Johan Castberg (21 September 1862 – 24 December 1926) was a Norwegian jurist and politician best known for representing the Radical People's Party (Labour Democrats).

The brother-in-law of Katti Anker Møller, the two were responsible for implementing the highly progressive Castberg laws, granting rights to children born out of wedlock.

[3] He had several brothers and sisters,[4] including the notable violinist Torgrim[5] and the politician Leif.

His paternal grandfather Peter Hersleb Harboe Castberg was a priest and politician.

[3] Parallel with the growing industrialisation of Norway, labor rights became a more central political question.

The organization of labourers had been growing since the 1880s, and their representatives demanded an expanded social policy, as opposed to the philanthropic individual taking care of his employees.

Workers' societies (arbeidersamfunn), which were not trade unions in the traditional sense, had existed since the middle of the nineteenth century.

However, in 1885 a nationwide umbrella organization was founded, with the name De forenede norske Arbeidersamfund, DFNA.

However, as local chapters of DFNA left to join the Labour Party instead, the support of this organization waned in some districts, especially in cities.

[9][10] In 1900 Castberg was elected to the Norwegian Parliament for the first time, representing the constituency of Kristians Amt.

[11] His power base being the workers' societies, Castberg was among the most radical members of the Liberal Party parliamentary group.

Their platform being based on promises of negotiations with Sweden concerning Norwegian rights to consulates, they got the Prime Minister position in the second cabinet Hagerup.

The summer of 1905 saw a Swedish reaction, coercing Norway to undergo negotiations in Karlstad.

[15] While a plebiscite on accepting Prince Carl of Denmark as King of Norway was held in November 1905, Castberg advocated the Republican form of government.

During this period his seat in Parliament was taken by Nils Gregoriussen Skilbred, though Castberg had not been re-elected in 1909.

[11] He left the cabinet on 20 April 1914,[3] due to disagreements with Prime Minister Gunnar Knudsen.

[18] Johan Castberg died in December 1926 in Oslo,[6] before the end of his seventh parliamentary term.