Thomas Wiliems (born in Ardda'r Mynaich in Arllechwedd, Wales possibly on 20 April 1545 or 1546; died in or before 13 August 1623) was a Welsh-language antiquarian.
He attended Brasenose College, Oxford for a time, but clear records of his studies there are lacking because of confusion with other similarly named students, and it seems he did not take a degree.
However, Wiliems became a recusant, converting to Roman Catholicism, after which he worked as a physician (drawing on his extensive book-learning: qualifications as such were not required at the time).
He died before August 13, 1623, at which time his uncle Sir John Wynn of the neighbouring Gwydir Castle, inscribed his name on the manuscript of Wiliem's dictionary as its owner.
His copying included:[1] It is rumoured that Wiliems was involved in the Gunpowder plot and, in warning his relative John Wynn not to go to the State Opening of Parliament in 1605, was responsible to either a smaller or greater extent for the suspicions which ultimately caught Guy Fawkes.