A portable application (portable app), sometimes also called standalone software, is a computer program designed to operate without changing other files or requiring other software to be installed. In this way, it can be easily added to, run, and removed from any compatible computer without setup or side-effects.

In practical terms, a portable application often stores user-created data and configuration settings in the same directory it resides in. This makes it easier to transfer the program with the user's preferences and data between different computers. A program that doesn't have any configuration options can also be a portable application.

Portable applications can be stored on any data storage device, including internal mass storage, a file share, cloud storage or external storage such as USB drives, pen drives and floppy disks—storing its program files and any configuration information and data on the storage medium alone. If no configuration information is required a portable program can be run from read-only storage such as CD-ROMs and DVD-ROMs. Some applications are available in both installable and portable versions.

Some applications which are not portable by default do support optional portability through other mechanisms, the most common being command-line arguments. Examples might include /portable to simply instruct the program to behave as a portable program, or --cfg=/path/inifile to specify the configuration file location.

Like any application, portable applications must be compatible with the computer system hardware and operating system.

Depending on the operating system, portability is more or less complex to implement; to operating systems such as AmigaOS, all applications are by definition portable.

Most portable applications do not leave files or settings on the host computer or modify the existing system and its configuration. The application may not write to the Windows registry or store its configuration files (such as an INI file) in the user's profile, but today, many portables do; many, however, still store their configuration files in the portable directory. Another possibility, since file paths will often differ on changing computers due to variation in drive letter assignments, is that portable applications may store them in a relative format. While some applications have options to support this behavior, many programs are not designed to do this. A common technique for such programs is the use of a launcher program to copy necessary settings and files to the host computer when the application starts and move them back to the application's directory when it closes.

An alternative strategy for achieving application portability within Windows, without requiring application source code changes, is application virtualization: An application is "sequenced" or "packaged" against a runtime layer that transparently intercepts its file system and registry calls, then redirects these to other persistent storage without the application's knowledge. This approach leaves the application itself unchanged, yet portable.