Deathworld is the name of a series of science fiction novels by American writer Harry Harrison, including the books Deathworld (first published 1960, serialized in Astounding Science Fiction), Deathworld 2 (1964, initially titled The Ethical Engineer and serialized in Analog) and Deathworld 3 (1968, serialized in Analog as The Horse Barbarians), plus the short story "The Mothballed Spaceship" (1973, written as part of a tribute to John W. Campbell). The central hero is a gambler who becomes involved with colonists of an extremely hostile planet.

Deathworld centers on Jason dinAlt, a professional gambler who uses his erratic psionic abilities to tip the odds in his favor. While visiting the planet Cassylia, he is challenged by a man named Kerk Pyrrus (an ambassador of the planet Pyrrus) to turn a large amount of money into an immense sum by gambling at a government-run casino. He succeeds and survives the planetary government's desperate efforts to take back the money. Bothered that he may finally have met someone superior to him, he decides to accompany Kerk to Pyrrus, despite being warned that it is the deadliest world ever colonized by humans.

There have been numerous supernovae in the region, meaning that planets in the area are rich in valuable radioactive ores, but Pyrrus is the only even marginally habitable one, and thus the only one that can support sustained mining operations. Pyrrus is no paradise. It has a gravity of 2 g; its 42° axial tilt creates severe weather; it has frequent earthquakes and volcanic eruptions; two large moons generate tides of up to 30 meters; and finally, there are high levels of radiation.

Everything on the planet is deadly to humans. The large animals are strong enough to destroy small vehicles, while the small ones have neurotoxic venom. Even the plants are deadly. All microorganisms consume insufficiently protected tissue as quickly as acids. On top of all this, life evolves so quickly that even Kerk and his Pyrran crew have to be retrained upon their return in order to survive.

Because of this harsh environment, the settlers are engaged in a ceaseless struggle to survive, which – despite generations of acclimation and a training regime harsher than that of ancient Spartans – they are losing. The money Jason won is used to buy desperately needed weapons.

While acclimating to the harsh planet, Jason turns his attentions toward solving the planet's mysteries and saving the faltering colony. The few surviving historical records show Jason that the settlers' numbers have dwindled since the planet was first colonized, and they are now restricted to a single settlement. Extrapolating backward, it is clear to Jason that the flora and fauna were once far less hostile to humans. Jason also learns of greatly despised "grubbers", humans living outside the city, with whom the Pyrrans grudgingly trade hardware for increasingly necessary food.

After several weeks, Jason leaves the city in search of the grubbers, who live in harmony with the harsh environment. They practice what many would consider suicidal forms of animal husbandry, with the assistance of their telepathic "talkers". Jason is able to earn their trust by demonstrating his own abilities. The outsiders' knowledge of the initial colonization effort is even more intriguing than that of the city dwellers. Not long after their arrival, animals suddenly began attacking the city, and have not stopped since. However, a number of colonists lived outside the city. Though they still found the planet incredibly harsh, they never suffered such attacks. The grubbers are their descendants. The two factions despise each other. The grubbers hate the city Pyrrans, or "junkmen", for cutting them off from space and refusing to trade food or ore for scientific knowledge or advanced technology, particularly medicine. The junkmen hate the grubbers for thriving while they are dying.

While studying the grubber community, he notices an anomaly—though the life-forms throughout the area are dangerous, they are nowhere near as lethal as the ones around the city. Some grubbers theorize that the initial schism was a disagreement over the city's location, in which the ancestors of the grubbers abandoned the dangerous ground in favor of their current homes. Jason has the grubbers guide him back to the city, so he can see it from the outside. There his psionic senses confirm his hypothesis. Every species of native flora and fauna is psionic, and all life around the city is telepathically "shouting" the same thing: "KILL THE ENEMY!" Pyrrus' biosphere is intentionally attacking the city.