运行时库（runtime library，又称运行期库），在计算机进程设计领域中，是指一种被编译器用来实现编程语言内置函数以提供该语言进程运行时（执行）支持的一种特殊的计算机进程库。这种库一般包括基本的输入输出或是内存管理等支持。它是一群支持正在运行程序的函式，与操作系统合作提供诸如数学运算、输入输出等功能，让程序写作者不需要“重新发明轮子”，并善用操作系统提供的功能。

运行时库由编译器决定，以面向编程语言，提供其最基本的执行时需要。比如Visual Basic需要复杂的运行时库支持而C的运行时库则相对简单。当然这还是由编译器厂商决定的。运行时库中的函数可能对进程员透明，也可能不透明。这也是由编译器厂商忖度语言执行环境的需求而决定的。

In computer programming, a runtime library is a set of low-level routines used by a compiler to invoke some of the behaviors of a runtime environment, by inserting calls to the runtime library into compiled executable binary. The runtime environment implements the execution model, built-in functions, and other fundamental behaviors of a programming language. During execution (run time) of that computer program, execution of those calls to the runtime library cause communication between the executable binary and the runtime environment. A runtime library often includes built-in functions for memory management or exception handling. Therefore, a runtime library is always specific to the platform and compiler.