外部装置（英语：Peripherals 或 英语：Peripheral Devices），又叫周边设备、外部设备（简称外设），是能够通电并正常运行的计算机的非必须硬件设备。它们可以独立或半独立工作而不依赖于计算机，通常可以扩展或提高所接入的计算机的功能或性能。包括输入输出设备和部分存储设备。如键盘、鼠标、扫描仪、摄像头、显示器、打印机、光盘驱动器、UPS 不间断电源和其它闪存盘、储存卡读卡器、数字相框、MP3/MP4 播放器、移动硬盘等 USB 设备或便携设备。正常的计算机即使去除这些设备，也能开机进入准备工作的就绪状态（虽然对用户来说，操作这样的计算机可能变得困难或不可行）。

A peripheral is a "device that is used to put information into or get information out of the computer."

There are three different types of peripherals: input devices, which interact with or send data from the user to the computer (mice, keyboards, etc.), output devices, which provide output to the user from the computer (monitors, printers, etc.), and input/output devices that perform both functions. Touchscreens are an example that combines different devices into a single hardware component that can be used both as an input and output device.