A Princess of Mars is a science fantasy novel by American writer Edgar Rice Burroughs, the first of his Barsoom series. It was first serialized in the pulp magazine All-Story Magazine from February–July, 1912. Full of swordplay and daring feats, the novel is considered a classic example of 20th-century pulp fiction. It is also a seminal instance of the planetary romance, a subgenre of science fantasy that became highly popular in the decades following its publication. Its early chapters also contain elements of the Western. The story is set on Mars, imagined as a dying planet with a harsh desert environment. This vision of Mars was based on the work of the astronomer Percival Lowell, whose ideas were widely popularized in the late 19th and early 20th centuries.

The Barsoom series inspired a number of well-known 20th-century science fiction writers, including Jack Vance, Ray Bradbury, Arthur C. Clarke, Robert A. Heinlein, and John Norman. The series was also inspirational for many scientists in the fields of space exploration and the search for extraterrestrial life, including Carl Sagan, who read A Princess of Mars when he was a child.

John Carter, a Confederate veteran of the American Civil War, goes prospecting in Arizona with his companion James K. Powell immediately after the war's end. Having struck a rich vein of gold, he runs afoul of the Apaches who kill Powell. While attempting to evade pursuit by hiding in a sacred cave, he is mysteriously transported to Mars, called "Barsoom" by its inhabitants. Carter finds that he has great strength and superhuman agility in this new environment as a result of its lesser gravity and lower atmospheric pressure. He soon falls in with the Tharks, a nomadic tribe of Green Martians, as the planet's warlike, six-limbed, green-skinned inhabitants are known. Thanks to his strength and martial prowess, Carter rises to a high position in the tribe and earns the respect and eventually the friendship of Tars Tarkas, one of the Thark chieftains.

The Tharks subsequently capture Dejah Thoris, Princess of Helium, a member of the humanoid red Martian race. The red Martians inhabit a loose network of city-states and control the desert planet's canals, along which its agriculture is concentrated. Carter rescues Dejah Thoris from the green men in a bid to return her to her people.

Subsequently, Carter becomes embroiled in the political affairs of both the red and green Martians in his efforts to safeguard Dejah Thoris. He eventually raises a horde of Tharks and other tribes, and leads them against the city-state of Zodanga, the historic enemy of Helium. Winning Dejah Thoris' heart, he becomes Prince of Helium, and the two live happily together for nine years. However, the sudden breakdown of the Atmosphere Plant that sustains the planet's waning air supply endangers all life on Barsoom. In a desperate attempt to save the planet's inhabitants, Carter uses a secret telepathic code to enter the factory, bringing an engineer along who can restore its functionality. Carter then succumbs to asphyxiation, only to awaken back on Earth, left to wonder what has become of Barsoom and his beloved.

Burroughs began work on A Princess of Mars in the summer of 1911 when he was 35. He wrote most of the first half of the novel while working for his brother in a stationery company, penning the words on scratch pads produced by the business. He had been struggling for some time to establish himself as a businessman, so far with little success, and with a wife and two children to support, turned to writing in desperate need of income. Despite failure in his business affairs, he had accumulated a wealth of unusual experiences from working in a variety of jobs which had brought him into contact with miners, soldiers, cowboys, and Native Americans.

While writing A Princess of Mars, Burroughs initiated what soon became a regular writing tool: maintaining worksheets relating to the piece he was completing. The sheets included start and end dates of writing, titles of chapters, and characters. By August 11, 1911, he had completed a large section of the novel. He was apprehensive about revealing what he was working on, and told only his wife that he was doing so. He still hoped to find business success, and thought the tale to be indicative of a childish nature, and so outlandish that potential business contacts would think him ungrounded if they discovered what he was working on. At this point he had already decided to adopt the pen name of "Normal Bean", an attempt to suggest that despite the incredible nature of his story, he was still a sane, reliable character. He struggled to find an appropriate title for the novel: My First Adventure of Mars, The Green Martians, and Dejah Thoris, Martian Princess were all early attempts to solve this problem.

Before completing the novel, he considered options for publishing, and realized he knew little about this world or how to submit a story. Because he liked and was familiar with The All-Story magazine, he submitted 43,000 words to the editor under the title "Dejah Thoris, Martian Princess". His cover letter explained that he thought he could produce another two parts of similar length. The Managing Editor of the magazine, Thomas Newell Metcalf, wrote back on August 24, 1911, to offer some criticisms of the pacing and focus of the tale, and suggested omitting the chapter "Sola Tells Me Her Story" (it was restored in the novel); he suggested that if Burroughs could finish the novel at under 70,000 words, he (Metcalf) would consider publishing it.