A Deepness in the Sky is a science fiction novel by Vernor Vinge. It is a loose prequel to his novel A Fire Upon the Deep, which is set 20,000 years later. It was nominated for the 1999 Nebula Award for Best Novel and won the 2000 Hugo Award for Best Novel. The first Tor mass market paperback edition is 775 pages.

The book deals with the discovery of an intelligent alien species on a planet orbiting the bizarre, appropriately named On/Off star, which spends over 200 of every 250 years completely dormant, releasing almost no energy. During this period, the planet freezes and its inhabitants go into hibernation.

The planet's inhabitants, named Spiders, are going through a period technologically very similar to Earth's 20th century. Two expeditions set out to trade and exploit, the Qeng Ho traders (presumably named after Cheng Ho) and the Emergents, an autocratic culture that literally enslaves the mind. As they approach the star, a surprise attack by the Emergents leaves both fleets crippled, and they are forced to cooperate and to wait for the Spider civilization to achieve a greater technological maturity to help them refurbish their ships.

The book discusses some of the problems of trying to maintain an interstellar trading culture without access to superluminal travel. One of the interesting concepts in the book is that the Qeng Ho measure time only in terms of seconds since the notion of days, months, and years has no usefulness between various star systems. The timekeeping system is based on the Unix epoch and the story frequently uses terms such as kiloseconds and megaseconds.

The only real connection with A Fire Upon the Deep is the character of Pham Nuwen, the "Programmer-at-Arms", who appears in both books.

This is the first Vinge work which introduces localizers. These are tiny devices which can contain a simple processor, sensors, and short-range communications. Vinge explores how mesh networking of these devices can be put to use in ways quite different from traditional computer networks.