Sandfly or sand fly is a colloquial name for any species or genus of flying, biting, blood-sucking dipteran (fly) encountered in sandy areas. In the United States, sandfly may refer to certain horse flies that are also known as "greenheads", or to members of the family Ceratopogonidae. The bites usually result in a small, intensely itchy bump or welt, the strength of which intensifies over a period of 5-7 days before dissipating. Moderate relief is achieved with varying success through the application of over the counter products such as Benadryl (ingested) or an analgesic cream such as After Bite. Outside the United States, sandfly may refer to members of the subfamily Phlebotominae within the Psychodidae. Biting midges (Ceratopogonidae) are sometimes called sandflies or no-see-ums. New Zealand sandflies are in the genus of sand fly Austrosimulium, a type of black fly.

Lutzomyia is a genus of phlebotomine sand flies consisting of nearly 400 species, at least 33 of which have medical importance as vectors of human disease. Species of the genus Lutzomyia are found only in the New World, distributed in southern areas of the Nearctic and throughout the Neotropical realm. Lutzomyia is one of the two genera of the subfamily Phlebotominae to transmit the Leishmania parasite, with the other being Phlebotomus, found only in the Old World. Lutzomyia sand flies also serve as vectors for the bacterial Carrion's disease and a number of arboviruses.