William Sydney De Mattos (10 January 1851 – fl.1909) was a British socialist activist.
Born in Clapham, De Mattos graduated in mathematics from Trinity College, Cambridge,[1] becoming captain of Cambridge University A.F.C. He then worked as a private tutor for the university and the British Army. He became interested in radical politics, and eventually became a socialist.[2]
In 1874, De Mattos married Katharine Stevenson, an author and the cousin of Robert Louis Stevenson, but the marriage was soon in trouble, and the two separated early in the 1880s.[3]
De Mattos joined the Democratic Federation, and then the Fabian Society, for which he lectured widely, and was a founder of many local Fabian societies. For some years, he served on the society's executive committee. His travels inspired him to launch "red van" campaigns, where a group of socialist lecturers toured rural areas, giving speeches from a van painted red. He also wrote articles for the Sunday Chronicle, the Weekly Times and Echo and the Dispatch.[2]
De Mattos was associated with the General Railway Workers' Union, working for a time as a union organiser, and also becoming a trustee of the union.[2] By the end of the 1890s, he had emigrated to Canada, where he lived until at least 1909.[3]