William Frederick Denning, FRAS (25 November 1848 – 9 June 1931) was a British astronomer who achieved considerable success without formal scientific training.

Denning devoted a great deal of time to searching for comets, and discovered several including the periodic comet 72P/Denning-Fujikawa and the lost comet D/1894 F1. The latter was the last comet discovered on British soil until the discoveries of George Alcock.

Denning also studied meteors and novas, discovering Nova Cygni 1920 (V476 Cyg). He won the Gold Medal of the Royal Astronomical Society in 1898.

Craters on Mars and the Moon, and the asteroid (71885) Denning are named in his honor.

Mr. Denning was elected a Corresponding Member of the Astronomical and Physical Society of Toronto on 1891-11-16.