Commander Keen in Goodbye, Galaxy (stylized as Goodbye, Galaxy!) is a two-part episodic side-scrolling platform video game developed by id Software and published by Apogee Software in 1991 for DOS. It consists of the fifth and sixth episodes of the Commander Keen series, though they are numbered as the fourth and fifth, as Commander Keen in Keen Dreams is not part of the main continuity. The game follows the titular Commander Keen, an eight-year-old child genius, as he first journeys through the Shadowlands to rescue the Gnosticenes so they may ask the Oracle how the Shikadi plan to destroy the galaxy, and then through the Shikadi's Armageddon Machine to stop them. The two episodes feature Keen running, jumping, and shooting through various levels while opposed by aliens, robots, and other hazards.

After the success of Commander Keen in Invasion of the Vorticons, the developers of the game, including programmers John Carmack and John Romero, designer Tom Hall, and artist Adrian Carmack, left their jobs at Softdisk to found id Software. After making a prototype game in Keen Dreams to develop new ideas such as gameplay changes, graphical enhancements, and artistic improvements, the team worked on making a sequel trilogy of episodes from June to December 1991. During development the last episode was split off to be released as a stand-alone game, Commander Keen in Aliens Ate My Babysitter, with the remaining two episodes produced as a pair instead of a trilogy.

Goodbye, Galaxy did not initially sell as well as the first trilogy, which was attributed by the publisher and designer to its lack of a third episode hurting it given the shareware model of publishing. The pair of episodes has since been referred to as "Apogee's hottest sellers", however. Reviews of the series praised the challenge and humor in the graphics and gameplay, especially in Goodbye, Galaxy. Although another Keen game was planned, during development id Software began work on Wolfenstein 3D, and its success, along with the development of Doom, led id to not develop any further Keen games.

The two episodes of Commander Keen in Goodbye, Galaxy are side-scrolling platform video games: the majority of the game features the player-controlled Commander Keen viewed from the side while moving on a two-dimensional plane. The player can move left and right and can jump, and can use a pogo stick to bounce continuously and jump higher than they can normally with the correct timing. The levels are composed of platforms on which Keen can stand, viewed from slightly above so as to give a pseudo-3D effect, and some platforms allow Keen to jump up through them from below, while others feature fireman's poles that Keen can climb up or down. Keen can also grab onto the edge of platforms and pull himself up. Once entered, the only way to exit a level is to reach the end, though unlike in the first trilogy of episodes, Commander Keen in Invasion of the Vorticons, the player can save their game at any point. In between levels the player travels on a two-dimensional map, viewed from above; from the map the player can enter levels by approaching the entrance or save their progress in the game. Some levels are optional to enter and may be bypassed.

Both of the episodes contain a different set of enemies in their levels, which the player must stun or avoid. The first episode contains a variety of alien creatures, including large slugs, giant bouncing mushrooms, and flying bats, while the second contains different types of robots and alien energy beings. Levels can also include hazards, such as dart traps or fire; touching a hazard or most enemies causes Keen to lose a life, and the game is ended if all of Keen's lives are lost. The episodes feature three difficulty settings, which change the number and types of enemies present. While Vorticons contained a raygun and the in-between episode Commander Keen in Keen Dreams had pellets that temporarily stunned enemies, Goodbye, Galaxy features a stun gun that knocks out most enemies permanently using ammo found throughout the game. Different enemies take differing numbers of shots to kill, or in some cases are immune. The player can also find food items throughout the levels which grant points, with an extra life awarded with enough points. The first episode also contains water droplets which award an extra life for every 100 drops, and the second green drinks which do the same. There are also colored keycards which grant access to locked parts of levels, and items which instantly grant an extra life.

The game is broken up into two episodes: "Secret of the Oracle" and "The Armageddon Machine". In the first episode, having defeated the Grand Intellect in Invasion of the Vorticons and saved the Earth, eight-year-old child Billy Blaze is building a faster-than-light communications radio. Upon completing it, he hears a transmission announcing that a race of aliens known as the Shikadi are planning to destroy the galaxy. Donning his helmet as Commander Keen, he takes off in his spaceship—first stunning his parents with his neural stunner gun as they call him in to dinner—for the planet of Gnosticus IV, home of the Oracle and the Gnosticenes that tend it, to find out who the Shikadi are and how they plan to destroy the galaxy. Upon arriving, he is met by a Council Page, who tells him that the council of immortal Gnosticenes have been kidnapped by the Shikadi and taken "to the Shadowlands far to the west", leaving monsters and traps to guard them. Keen journeys through the outposts and temples of the Shadowlands, rescuing all of the Gnosticenes, and afterwards they turn on the Oracle machine. The Oracle tells Keen that the Shikadi are "shadow beings from the far side of the galaxy" who are building an Armageddon Machine at Korath III to blow up the galaxy and rebuild it as they wish afterwards. As the episode ends, Keen sets off to stop them.

In "The Armageddon Machine", Keen journeys through the giant Armageddon Machine space station, destroying the subsystems of the machine located in each level. Along the way, he learns that the Shikadi are being led by the "Gannalech". After Keen reaches the final level and destroys the Quantum Explosion Dynamo, the Shikadi flee the station, and either leave Korath III via their getaway ship or are left stranded, depending on whether Keen destroys a fuse in the secret level. Keen looks in the control room to find out why the Shikadi wanted to destroy the galaxy; there, he finds a note, written in the series' Standard Galactic Alphabet cypher, from the final boss of the Vorticons trilogy: his school rival Mortimer McMire, whose IQ is "a single point higher" than Keen's. The note tells Keen that the McMire killed in the previous game was an android replica, that the "Gannalech" is a mispronunciation of "Grand Intellect", his title from the first trilogy, and that the Armageddon Machine was itself a distraction, as McMire plans on destroying the universe instead. As Keen leaves the command center, the game screen focuses on an abandoned football helmet in the room like Keen's, with two Ms on it.

In October–December 1990, a team of employees from programming studio Softdisk, calling themselves Ideas from the Deep, developed the three-part video game Commander Keen in Invasion of the Vorticons. The group, who worked at Softdisk in Shreveport, Louisiana developing games for the Gamer's Edge video game subscription service and disk magazine, was composed of programmers John Romero and John Carmack, designer Tom Hall, artist Adrian Carmack, and manager Jay Wilbur. After the release of the game in December, and the arrival of the first US$10,500 royalty check from shareware publisher Apogee Software, the team quit Softdisk and started their own company, id Software. As part of the settlement with Softdisk—made because the team had created the game on their work computers, both in the office after hours and by taking the computers to John Carmack's house on the weekends—they agreed to make a series of games for Softdisk's Gamer's Edge subscription service. As the team began to explore creating another set of Commander Keen games, they made a prototype game for Softdisk, Commander Keen in Keen Dreams, to fulfill their obligations while also helping improve the next full set of Keen games.