Nicolas Dethier (29 September 1888 – 12 February 1976) was a Belgian trade unionist and politician.
Born in Beyne-Heusay, Dethier left school at the age of twelve and became a coal miner.
In 1906, he joined the Federation of Miners of Liege, and in 1908 he became the secretary of his local section of the union.
In 1954, Dethier was elected as vice-president of the Miners' International Federation, then in 1956 he succeeded as president.
In 1954, he was elected to the general council of the Belgian Socialist Party, and from 1954 to 1961 he served as a co-opted member of the Senate.