Any member of the insect family Simuliidae, comprising 300 species of small, humpbacked dipterans found worldwide. Usually black or dark gray, the blackfly has short mouthparts adapted for sucking blood. The females bite and are sometimes abundant enough to kill chickens and even cattle. Some species carry worms capable of causing human disease, including river blindness. In subarctic regions blackflies may be so numerous that human habitation is impossible.

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Blackfly, black-fly, or black fly can refer to:

• a common name for flies of the family Simuliidae.
• a common name for some aphids.
• Blackfly (TV series), the name of a former Canadian comedy series.
• The Black Fly Song, written by Wade Hemsworth.