The Lost Galaxy is the titular galaxy and the main story arc featured in the season Power Rangers Lost Galaxy, beginning in "Enter The Lost Galaxy" and ending with "Escape The Lost Galaxy".
With the help of the Keonta Spell, Deviot uses the power of the Galaxy Book open the portal and bring the Terra Venture space colony into the Lost Galaxy. The galaxy is home of the space pirate, Captain Mutiny and his Swabbies (along with Barbarax, Hexuba and the Titanisaur, the living vessel where Mutiny's Castle presides over). Mutiny and his group presides on a unknown desert planet, where he hordes slaves (presumably those who entered the Galaxy by accident or others residents of the universe) to collect jewels or other precious gems. In one episode he captures Leo and Damon and leaves them to die from exposure to his planet's sun, while in another, Mike infiltrated his slave camp to rescue the kidnapped colonists of Terra Venture along with the other slaves. Hexuba's mansion and graveyard are also located in the Galaxy, a fair distance from Mutiny's planet as he has to travel there using the Titanisaur. Hexuba used her graveyard to resurrect fallen enemies of the Galaxy Rangers as a way to defeat them, but her plan is foiled when Kai destroys her orb and she herself is destroyed by the Galaxy Megazord.
The Lost Galaxy is said to be the "most mysterious place in the universe" as people never go in but never return from it. During the colonists' stay in the Lost Galaxy, the systems and navigation by stars were rendered useless, as there were no familiar stars they could recognize and electronic systems were scrambled, as noted with a hand clock whose hands spun backwards. Plant life was also affected, as plants in a pot of sand regressed into nothingness. Prior to this, Stanton and Mike experience a vision of an old man warning them to leave the Galaxy or remain trapped forever.
When Captain Mutiny greets the stranded colony, Commander Stanton explains they were caught in a energy storm, to which the pirate is not surprised by. According to Mutiny, the portal to the Lost Galaxy opens approximately every one hundred years, bringing with it wanders who stumbled through the wormhole. Mutiny offers the colony a proposition of peace, saying that he would help them find their way back. Instead, the pirate sets Grunchor on the colony as a show of his power over the Galaxy and it's occupants.
Terra Venture eventually escapes the Lost Galaxy when Leo and Kai read the Keonta spell in reverse, causing a chain reaction which causes the Galaxy Book to transform into a portal out of the universe. They escape with the help of Mike as the Magna Defender, who sacrifices his powers to keep the portal open. Captain Mutiny follows them out of the Lost Galaxy with the prospects of conquering another galaxy, however, he and his castle are destroyed by Trakeena, who asserts her power over the galaxy.
- In the original script of Quasar Quest, Mirinoi was originally to be apart of the Lost Galaxy, but the concept was later retconned with episode "Escape The Lost Galaxy" and "Journey's End". The planet, complete with a different geography, is outside the Lost Galaxy and juxtaposed against different area of space altogether. As such, though the galaxy of first episode showcasing the planet sports a background similar to that of the Lost Galaxy, it should be noted that Mirinoi is outside the realm of that particular galaxy.
- Dino Thunder's episode "Legacy of Power", later contradicts this fact when Tommy states the Galaxy Rangers were "chosen to be the Power Rangers of the Lost Galaxy".