Gadfly can refer to:
- "Gadfly", a fly that annoys horses and other livestock, usually a horse-fly or a botfly
- Social gadfly, a person who upsets the status quo
- Gadfly (database), a relational database in the Python programming language
- The Gadfly, a novel about revolution in Italy published in 1897 by American author Ethel Lilian Voynich
- The Gadfly (1928 film), a Soviet film based on the novel
- The Gadfly (1955 film), a Soviet film based on the novel
- The Gadfly (1980 film), a Soviet film based on the novel
- The Gadfly (Zhukov opera), 1928 Russian opera by Mikhail Zhukov
- The Gadfly (Ziks opera), 1930 Russian opera by A. Ziks
- The Gadfly (opera), 1958 a Russian opera by Soviet composer Antonio Spadavecchia
- The Gadfly Suite, a musical suite by Dmitri Shostakovich for one of the films based on the novel The Gadfly by Ethel Lilian Voynich
- Gadfly (mythology), sent by Hera to torment Io
- The Education Gadfly, the weekly e‑bulletin of the Thomas B. Fordham Foundation
- Gadfly petrels, a group of petrels in the genera Bulweria and Pterodroma
- Gadfly Online, an online and print magazine
- HMS Gadfly, four ships of the Royal Navy
- SA‑11 "Gadfly", the NATO reporting name for a Russian surface-to-air missile system "9K37 Buk"
- The Gadfly (album), an album by Christian hip-hop group LPG