Browser hijacking is a form of unwanted software that modifies a web browser's settings without a user's permission, to inject unwanted advertising into the user's browser. A browser hijacker may replace the existing home page, error page, or search engine with its own. These are generally used to force hits to a particular website, increasing its advertising revenue.

Some browser hijackers also contain spyware, for example, some install a software keylogger to gather information such as banking and e-mail authentication details. Some browser hijackers can also damage the registry on Windows systems, often permanently.

While some browser hijacking can be easily reversed, other instances may be difficult to reverse. Various software packages exist to prevent such modification.

Many browser hijacking programs are included in software bundles that the user did not choose and are included as "offers" in the installer for another program, often included with no uninstall instructions, or documentation on what they do, and are presented in a way that is designed to be confusing for the average user, to trick them into installing unwanted extra software.

There are several methods that browser hijackers use to gain entry to an operating system. Email attachments and files downloaded through suspicious websites and torrents are common tactics that browser hijackers use.