William Sydney De Mattos (10 January 1851 – fl.1909) was a British socialist activist.
He then worked as a private tutor for the university and the British Army.
[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.
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.