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" (family Tabanidae), 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 (applied topically). Outside the United States, sandfly may refer to members of the subfamily Phlebotominae within the Psychodidae. Biting midgets (Ceratopogonidae) are sometimes called sandflies or no-see-ums (no-see-em, noseeum). New Zealand sandflies are in the genus of sand fly Austrosimulium, a type of black fly.

In the various sorts of sandfly only the female is responsible for biting and sucking the blood of mammals, reptiles and birds; the protein in the blood is necessary for the production of eggs, making the sandfly an anautogenous reproducer.

Some sandfly genera of the subfamily Phlebotominae are the primary vectors of leishmaniasis and pappataci fever; both diseases are confusingly referred to as sandfly fever. In Asia, Africa, and Europe, leishmaniasis is spread by sand flies of the genus Phlebotomus; in the Americas, the disease is spread by sandflies of the genus Lutzomyia. Belize and Honduras are notorious in the Caribbean for their sandfly populations and travel pages frequently warn tourists to bring bug spray containing high concentrations of DEET.