Out of the Silent Planet is a science fiction novel by the British author C. S. Lewis, first published in 1938 by John Lane, The Bodley Head. Two sequels were published in 1943 and 1945, completing the Space Trilogy.

While on a walking tour, the philologist Elwin Ransom is drugged and taken on board a spacecraft bound for a planet called Malacandra. His abductors are Devine, a former college acquaintance, and the scientist Weston. Wonder and excitement relieve his anguish at being kidnapped, but he is put on his guard when he overhears his captors discussing their plans to turn him over to the inhabitants of Malacandra as a sacrifice.

Soon after the three land, Ransom escapes and then runs off in terror upon first seeing the vaguely humanoid but alien sorns. In his wanderings, he finds that all the lakes, streams, and rivers are warm, that gravity is significantly lower than on Earth, and that the plants and mountains are all extremely tall and thin. After meeting a hross named Hyoi, a civilised native of a different species, Ransom becomes a guest for several weeks in Hyoi's village, where he uses his philological skills to learn the language. Discovering that gold (known as "sun's blood"), is plentiful on Malacandra, he discerns Devine's motive for making the voyage.

While out hunting, Ransom and his hrossa companions are told by an eldil, an almost invisible, angelic creature, that Ransom must go to meet Oyarsa, who is ruler of the planet, and indeed that he should already have done so. Shortly after, Hyoi is shot dead by Devine and Weston as they track Ransom, and Ransom is directed by the hrossa to comply with the eldil's instructions and cross the mountains to the cave of a sorn named Augray.

On the way Ransom discovers that he has almost reached the limit of breathable air and has to be revived by Augray with a flask of oxygen. The next day, carrying Ransom on his shoulder, Augray takes him across the bleak tableland and down into another river valley to Meldilorn, the island home of Oyarsa. There Ransom meets another species, a pfifltrigg who tells him about the beautiful houses and works of art that his people make in their native forests.

Ransom is led to Oyarsa, who explains that there is an Oyarsa for each of the planets in the Solar System. However, the Oyarsa of Earth – which is known as Thulcandra, "the silent planet" – has become "bent", or evil, and has been restricted to Earth after "a great war," on the authority of Maleldil, the ruler of the universe. Ransom is ashamed at how little he can tell the Oyarsa of Malacandra about Earth, and how foolish he and other humans seem to Oyarsa.

While the two are talking, Devine and Weston are brought in, guarded by hrossa, because they have killed three members of that species. Weston does not believe that Oyarsa exists and he is incapable of conceiving that the Malacandrians are anything but ignorant natives, exploitable and expendable. This emerges in the course of a long speech in which Weston justifies his proposed invasion of Malacandra on "progressive" and evolutionary grounds. Weston's motives are shown to be more complex than profit: he is bent on expanding humanity through the universe, abandoning each planet and star system as its resources are exhausted and it becomes uninhabitable. In Ransom's attempts at translation, the brutality and crudity of Weston's ambitions are laid bare.

While acknowledging that Weston is acting out of a sense of duty to his species and does not share Devine's greed for gold, Oyarsa tells Weston and Devine that he cannot tolerate their disruptive presence on Malacandra. He directs them to leave the planet immediately, despite very unfavourable orbital conditions. Oyarsa offers Ransom the option of staying on Malacandra, but Ransom decides he does not belong there either. The three men voyage back to Earth in the spaceship, and their 90 days' worth of air and other supplies nearly run out before they arrive.