The Principia Discordia is a Discordian religious text written by Greg Hill (Malaclypse the Younger) with Kerry Wendell Thornley (Lord Omar Khayyam Ravenhurst). The first edition was printed using Jim Garrison's Xerox printer in 1963.The second edition was published under the title Principia Discordia or How The West Was Lost in a limited edition of five copies in 1965. The phrase Principia Discordia, reminiscent of Newton's Principia Mathematica, is presumably intended to mean Discordant Principles, or Principles of Discordance.

The Principia describes the Discordian Society and its Goddess Eris, as well as the basics of the POEE denomination of Discordianism. It features typewritten and handwritten text intermixed with clip art, stamps, and seals appropriated from other sources.

While the Principia is full of literal contradictions and unusual humor, it contains several passages which propose that there is serious intent behind the work, for example a message scrawled on page 00075: "If you think the PRINCIPIA is just a ha-ha, then go read it again."

The Principia is quoted extensively in and shares many themes with the satirical science fiction book The Illuminatus! Trilogy by Robert Shea and Robert Anton Wilson. Wilson was not directly involved in writing the Principia.
Golden Apple, symbol of Eris, Our Lady of Discord

Notable symbols in the book include the Apple of Discord, the pentagon, and the "Sacred Chao", which resembles the Taijitu of Taoism, but the two principles depicted are "Hodge" and "Podge" rather than yin and yang, and they are represented by the apple and the pentagon, and not by dots. Saints identified include Emperor Norton, Yossarian, Don Quixote, and Bokonon. The Principia also introduces the mysterious word "fnord", later popularized in The Illuminatus! Trilogy; the trilogy itself is mentioned in the afterword to the Loompanics edition, and in the various introductions to the fifth editions.