In a computer, a barnacle is unwanted programming, such as adware or spyware, that is downloaded and installed along with a user-requested program. The term derives from the name of a crustacean that attaches itself to whales and boats, among other things. Like its marine counterpart, the computer barnacle can be difficult to eradicate.
Barnacles usually fall under the category of potentially unwanted programs (PUPs), a euphemistic term coined by McAfee to refer to programs that a user installs unintentionally, perhaps having unknowingly consented to their download.

Computer barnacles, like other spyware, can seriously affect computer performance. Unlike most spyware, however, they may also cause damage. Some barnacles interfere with the Winsock code that handles input/output requests for Internet applications in Windows operating systems. Winsock runs between a program (such as a browser) and the program that uses TCP/IP. Removal of this type of barnacle may corrupt Internet protocols and degrade network performance, in which case the user must reinstall the TCP/IP stack.

The term barnacle is closely related to drive-by download, which is programming that is downloaded without user consent and often without the user's knowledge that any download has occurred.