The Day of the Triffids is a post-apocalyptic 1951 novel by the English science fiction author John Wyndham. It was later made into a radio series (BBC, 1960), a motion picture (1962), and a TV series (BBC, 1981).

The triffid of the title is a very strange fictional plant, which has rudimentary animal-like behaviour: it is able to uproot itself and move, and possesses a whip-like poisonous sting.

The narrator recounts how the plant was discovered when he was a child, and quickly becomes established as a major crop due to its invaluable edible oils and proteins. After an aeroplane accident, seeds spread across the world and the plants become commonplace. Many households keep them as a curiousity, almost a garden pet, making sure to have the sting docked at regular intervals. In commercial exploitation, the stings are left intact as docking impairs the quality of the plant.

The book opens with the narrator in hospital, with his eyes bandaged after having been stung by a triffid. He discovers that while he had been blindfolded, an unusual meteor shower has blinded most people on Earth. He finds people in London struggling to stay alive, some cooperating, some fighting: after just a few days society is collapsing.

Meanwhile, triffids are quickly regrowing their stings. Undocked triffids in captivity break free. The handful of sighted survivors escape the general collapse, to be faced by the growing numbers of free, undocked triffids, which attack animals to later digest the bodies with their roots.

The possible intelligence of the plants is suggested; as well as the idea that they may have been genetically engineered by humans in the first place. The question of whether the meteor shower was in fact some sort of space-based weapons system which misfired is also raised.

In the film, the triffids arrive as spores in the same meteor shower that blinded almost everyone on earth, which at least simplifies the number of assumptions required to make the plot work. The film supplies a simplistic solution (Salt water dissolving them) to the triffid problem, while the novel and the BBC adaptations end with the triffids still dominant and a few human survivors grimly attempting to fight back.

In 2001 a sequel to the novel was published. It was entitled The Night of The Triffids and written by Simon Clark .