The Encyclopedia Galactica is a fictional or hypothetical encyclopedia of a future galaxy-spanning civilization, containing all the knowledge accumulated by a society with trillions of people and thousands of years of history.

It was first used by Isaac Asimov in his novel Foundation, where it is central to the first part of the plot. Various people have since used the same idea, both in science and in science fiction. Douglas Adams' The Hitchhiker's Guide to the Galaxy series frequently contrasts the Galactica with the presumably more popular Guide. For example, the introduction to the first book notes:

"In many of the more relaxed civilizations on the Outer Eastern Rim of the Galaxy, the Hitch Hiker's Guide has already supplanted the great Encyclopaedia Galactica as the standard repository of all knowledge and wisdom, for though it has many omissions and contains much that is apocryphal, or at least wildly inaccurate, it scores over the older, more pedestrian work in two important respects.
"First, it is slightly cheaper; and second, it has the words Don't Panic inscribed in large friendly letters on its cover."

Part XII of Carl Sagan's Cosmos: A Personal Voyage television series is titled Encyclopaedia Galactica. In it, Sagan suggests that such an encyclopedia might be broadcast into space and could possibly be received through radioastronomy (see SETI). He explores the idea of how different civilizations, including our own, might be described in such a work. In his version of the encyclopedia, each civilization has a "probability of survival" rating — for a hypothetical highly technologically advanced civilization utilizing Dyson Spheres, he suggests a probability of survival of 99% per 106 years. For Earth, he speculates that the probability of survival might be viewed by other civilizations as only 40% per 100 years, reflecting the Cold War conditions of the time the documentary was made.

There are similarities between the hypothetical Encyclopedia Galactica and the Wikipedia projects. Both are attempts to capture a broad range of human knowledge. Both are authored by legions of dedicated scholars. Because of this correlation, one may occasionally find references to characters or quotes from the Foundation novels in self-referencing Wikipedia articles. This is a subtle form of Wikipedia humor.

Other instances of such encyclopedias in fiction include:

The Repository of the Ancients, from Stargate SG-1
Adventures of Superman #617 (2003)