Sir William Fyncheden KS (died 1374) was an English justice.
In February 1355, he was appointed to investigate unauthorised Alienation of royal lands in 6 counties, including Nottinghamshire and Derbyshire.
In 1359, he investigated trespasses against the royal family and their tenants in Richmondshire, and in 1360 he was tasked with the inquiry as to whether the lands of Roger Mortimer were being held by the King or as part of Wales, to which he found the latter.
Around 1365, he inherited substantial lands, mostly in Yorkshire, from Sir William de Notton, Lord Chief Justice of Ireland, who was presumably his cousin.
His career continued to develop in the 1360s, with commissions of Oyer and terminer in Sussex, Huntingdonshire, Cambridgeshire, Kent and Gloucestershire.