`StringReader` is a class in the System.IO namespace of the C# programming language. It allows developers to read from a string as if it were a stream of characters. This is useful for processing text data without needing to read from a file or other external source. The `StringReader` class provides methods like `Read`, `ReadLine`, and `ReadToEnd`, enabling easy manipulation of string data. It is often used in scenarios where text needs to be parsed or analyzed, making it a valuable tool for developers working with string-based input.