A portable storage device (PSD) is a compact plug-and-play mass storage device designed to hold a large volume of digital data of any kind. This is slightly different from a portable media player, which is designed to only store music and video files that its internal reader softwares can play.

Most modern PSDs are dedicated solid state drives (SSD) that are connected to a computer and powered via USB ports. Some PSDs, usually those from before the wide adoption of SSDs, are modified hard disk drives via the installation of a disk enclosure, and require an additional AC adapter as the power required to operate the drive typically exceeds that can be provided by the USB port.

PSDs, while much bigger and heavier than ultracompact flash drives such as USB flash drives and memory cards, offer significantly more external storage capacities, yet are still convenient enough for carrying around when travelling or as a readily accessible offline backup storage option, especially in situations where online storage alternatives such as network-attached storage and cloud storage are unavailable, unreliable or unsafe.