An immutable data structure is a type of data structure that cannot be changed after it is created. This means that once you create an instance of an immutable data structure, you cannot modify its contents, such as adding, removing, or altering elements. Instead, any changes result in the creation of a new instance, preserving the original data. Immutable data structures are commonly used in programming languages like Haskell and Scala, as well as in functional programming paradigms. They offer benefits such as easier reasoning about code, improved safety in concurrent environments, and reduced chances of unintended side effects.