Gahagan is a surname of which is derived from Geoghegan. Notable people with the surname include:
Dennis Gahagan (c. 1817–?), early settler in San Diego, California Helen Gahagan Douglas (1900–1980), American actress and politician James Gahagan (1927–1999), American painter John Gahagan (born 1958), Scottish footballer Lawrence Gahagan (d.1820), Irish-born sculptor Sebastian Gahagan (1779–1838), Irish-born sculptor
See also Gahagan Mounds Site, a Caddoan Mississippian culture mound site in Red River Parish, Louisiana, US Geoghegan, the surname which Gahagan derives from