Flann is both an English surname and an Irish masculine given name, but has also been used as a feminine given name. It might refer to:
Flann Fína mac Ossu, another name for King Aldfrith of Northumbria (died 704 or 705) Flann mac Lonáin (died 896), Irish poet Flann Sinna (died 916), also called Flann mac Maíl Sechnaill, High King of Ireland Flann Mainistrech (died 1056), Irish scholar Flann O'Brien, a pen name used by Irish writer Brian O'Nolan (1911–1966)
FLANN, an acronym for Fast Library for Approximate Nearest Neighbors, is a C++ library for approximate nearest neighbor search in high-dimensional spaces.