Konrad Mathias Nordahl (25 September 1897 – 22 May 1975) was a Norwegian trade unionist and politician for the Labour Party. He was the leader of the Norwegian Confederation of Trade Unions from 1939 to 1965, and an MP from 1958 to 1965.
He was born in Laksevåg, then in a part of the municipality of Askøy. At the age of two, he lost his mother and was raised by his uncle and aunt as foster parents; he was then given the surname Nordahl instead of Johannessen. He joined the Norwegian Labour Party in 1912, and a trade union in 1915. He had secretary jobs for the Labour Party and its youth wing, and became a central board member of the Young Communist League of Norway in 1923. In the same year the organization seceded from the Labour Party, and became the young wing of a new Communist Party of Norway. Nordahl was a Communist Party member until 1927, and in 1929 he rejoined the Labour Party.
In 1923 Nordahl had married Constance Hole (1897–1986) and moved to Bergen, where he found work in a workshop. He joined the Norwegian Union of Iron and Metal Workers, and became national chairman in 1931. Already in 1934 he became vice chairman of the Norwegian Confederation of Trade Unions. He was promoted to chairman in 1939 when Olav Hindahl became a cabinet member. At the same time he joined the Norwegian Labour Party central board, where he remained until 1965. He was also a member of the executive committee of Oslo city council from 1937 to 1947.