Charles Joseph Gahan (20 January 1862 – 21 January 1939) was an Irish entomologist.
He was born on 20 January 1862 at Roscrea County Tipperary, Ireland. His father, Michael Gahan was the Master of Erasmus Smith's School in Tipperary. He was educated first at Queens College Galway, where he achieved distinction, and then at the Royal School of Mines in Kensington. In 1882 he was awarded a medal and prizes as the best biological student of the session. In 1886 he joined the British Museum (Natural History) as an assistant in the Department of Zoology where he became Keeper in the then newly formed Department of Entomology in 1913. An expert on beetles, especially Cerambycidae, he wrote the 1906 volume of The Fauna of British India, Including Ceylon and Burma on that group. Honorary Secretary of the Entomological Society of London in 1899-1900 and was president 1917-1918. Married Annie Woodward in 1887. He retired in 1920 and lived at Mouth Aylsham in Norfolk and died at Aylsham on 21 January 1939. He became the first person to describe Rosenbergia exigua in 1888.