William Filby (born c. 1557/1560 - 30 May 1582) was an English Roman Catholic priest.

Educated at Lincoln College, Oxford, he was admitted to the seminary at Reims on 12 October 1579.

He was ordained priest at Reims on 25 March 1581, and shortly after left for the English mission.

[2] He was arrested in July, along with Edmund Campion at Lyford Grange, the house of a certain Francis Yate, then in Berkshire, and committed to the Tower of London, removed 14 August to the Marshalsea, and thence back to the Tower again.

Filby was asked his opinion of Nicholas Sanders' landing with Spanish troops in County Kerry the previous September.