Barrier Ninjutsu (結界忍術, Kekkai Ninjutsu; Literally meaning "Barrier Ninja Techniques") is the incorporation of barriers into techniques. Essentially these techniques allow the users to manipulate barriers. The user can erect barriers that can be used to protect themselves, or to trap an opponent, amongst other various uses.
There are also several species of animals that make use of barrier techniques such as the Gourd Toad of Mount Myōboku which tiles the Barrier: Toad Gourd Prison. Barriers have also been incorporated into the form of tags for both offensive and defensive purposes.
The strength and efficiency of barriers seemingly depends on the type of technique used to create the barrier itself, and the ability of others to perceive, enter, or destroy the barrier. The Thirty-Six Layer Self-Repairing Barrier, created by a synchronised array of multiple shinobi, is supposedly able to contain a jinchūriki and its tailed beast, but has been shown to be unable to handle the combined efforts of two jinchūriki from breaking out. However, even the most powerful barriers, intended to prevent detection by outsiders, can be circumvented by the use of Space–Time Ninjutsu, such as Tobi's Kamui, which he used to gain access to the place where Kushina Uzumaki was giving birth to Naruto Uzumaki.
Barriers can be used for a variety of purposes, such as to prevent outsiders from entering, as with the Five-Seal Barrier; to prevent those inside from leaving the confines of the barrier, and even to prevent those trapped within from using certain techniques, as was done to stop Dan Katō from utilising the Spirit Transformation Technique by erecting the Four Violet Flames Formation around him.
Some Barrier Ninjutsu seems to have a tie-in with Space–Time Ninjutsu, by manipulating the very barrier of space-time continuum itself: examples include the inside of the Toad Gourd Prison, that is a barrier of a different world, while Kamui transfers the target to the other dimension and the Space–Time Barrier, used through the Flying Thunder God Kunai, allows to warp the target to another kunai's location.