The Guardian of Forever is a time portal portrayed in the fictional universe of Star Trek. The Guardian is a sentient entity, both machine and being, which can open time portals to any point in history on any world in the universe. The first appearance of the Guardian was in the Star Trek: The Original Series episode "The City on the Edge of Forever". In that episode, the voice of the Guardian was provided by Bartell LaRue .

No one knows who built the Guardian, although analysis of the ruins on its homeworld suggests it may be billions of years old. The Guardian is able to speak to anyone who asks it the right question, although it is usually very evasive in its responses (if it deigns to speak at all). The Guardian can detect changes in the timeline, but typically provides its users little help in figuring out how to change it back.