William John Hardy (29 September 1857 – 17 July 1919) was an English archivist and antiquarian.
He was born in London, the younger son of Sir William Hardy.
This work and that of George Herbert Fowler for Bedfordshire proved widely influential.
He edited State Papers, and Documents Illustrative of English Church History with Henry Gee.
He also edited Middlesex and Hertfordshire Notes and Queries from 1895 to 1898, and the first six volumes of The Home Counties Magazine, 1899 to 1904.