Magnus Nilssen (18 July 1871 – 20 November 1947) was Norwegian politician for the Labour and Social Democratic Labour parties.
He was born in Lillehammer as a son of shoemaker Mathias Nilssen (1834–1920) and his wife Eline Pedersen (1835–1918). He was a first cousin of Marcus Halfdan Kastrud. He finished his appreniceship as a goldsmith in 1889, and moved to Kristiania in the same year. He was a secretary in his local trade union from 1891 to 1892 and treasruer in 1893. He was also member of the socialist youth club Friheden in both Kristiania and Sarpsborg (where he lived in 1894). He started his own goldsmith business in 1897. In November the same year he married Inga Marie Ravneberg.
He joined the Norwegian Labour Party, and became a member of the central board in 1900. From 1901 to 1918 he was the party secretary. He lost out when "the new direction" became dominant in 1918. "The new direction" had tried to replace him with Alfred Madsen at the national convention in 1915. In 1921 he quit the central board to co-found a new party, the Social Democratic Labour Party of Norway. He chaired this party throughout its existence, from 1921 to 1927. He took part at the founding congress of the Labour and Socialist International in 1923; the other Norwegian delegates were Arne Magnussen, Michael Puntervold and Olav Kringen.
Nilssen was a member of Kristiania (Oslo) city council from 1902 to 1910, 1914 to 1919 and 1926 to 1928. To the Parliament of Norway he was elected for the first time in the Norwegian parliamentary election, 1906. He represented his city. He was re-elected eight times, to serve a total of nine terms in Parliament. There was a hiatus between 1922 and 1927; he was never elected to Parliament for the Social Democratic Labour Party. He served as President of the Lagting from 1919 to 1921 and Vice President of the Storting from 1935 to 1945. After the reunion of Labour and Social Democratic Labour in 1927, he rejoined the Labour Party central board, this time as deputy party leader. The party had two deputy leaders—Nilssen and Edvard Bull, Sr.—to accommodate different wings within the party. Nilssen also served as Minister of Labour in the two-week Hornsrud's Cabinet between January and February 1928.