In the science fiction universe of Star Trek, the Great Barrier is an energy field that surrounds the Milky Way Galaxy. Also referred to as "The Galactic Barrier", the energy field completely encompasses the galactic disk and prevents conventional starship travel beyond the edge of the galaxy.

A non-canon theory has suggested the barrier is not meant to keep explorers "in" the galaxy, but to keep something from beyond the galaxy "out". It is also uncertain whether or not the energy barrier is a natural or artificially created phenomena.

Although numerous attempts to travel outside the galaxy have been made, with various degrees of success, the barrier usually damages or destroys ships that try to pass through it. It also has psychoactive effects as some personnel gain powerful telekinetic and telepathic abilities if traveling near the barrier.

The barrier appears as a pinkish band of light, and upon entering it, a ship is surrounded by dancing clouds of colorful energy. Inside the barrier, a ship's power and engines shut down and navigational systems become erratic.

The Great Barrier made its first appearance in the Star Trek: The Original Series episode "Where No Man Has Gone Before" where the crew of the USS Enterprise had to deal with those affected by the psychic powers of the field - as did the crew of the ill-fated SS Valiant which had encountered the same barrier, two centuries earlier.

It was also encountered again in the episode "Is There in Truth No Beauty?" where the Enterprise becomes lost within the barrier and is saved by the incredible navigational skills of an alien passenger.

The barrier had also damaged an alien ship from outside the galaxy, in the episode "By Any Other Name", stranding the aliens in our galaxy until they try to steal the Enterprise and return to their home galaxy.

There is also a second Great Barrier at the core of the Milky Way, which was first revealed in the motion picture Star Trek V: The Final Frontier. This energy barrier at the core was thought to be impenetrable like the one at the edge of the galaxy, however it was revealed that only mankind's natural fear of the unknown was keeping those from entering it.