Immutable.js is a JavaScript library designed to create immutable data structures. This means that once a data structure is created, it cannot be changed. Instead of modifying existing data, any updates result in a new version of the data structure, which helps prevent unintended side effects in applications. By using Immutable.js, developers can improve performance and simplify state management in applications, especially in frameworks like React. The library provides various data types, such as lists, maps, and sets, that are optimized for efficient updates and comparisons, making it easier to work with complex data in a predictable manner.