Domain hijacking or domain theft is the process by which registration of a currently registered domain name is transferred without the permission of its original registrant, generally by exploiting a vulnerability in the domain name registration system.

Domain names expire after a fixed period of time and become available to the public. If their original owner allows them to expire even momentarily, they may be immediately purchased by another party. Although this may be frustrating for a domain owner who is slow to renew, generally the new purchaser is not considered liable in this scenario and the original owner has no recourse.

The most common tactic used by a domain hijacker is to use acquired personal information about the actual domain owner to impersonate them and persuade the domain registrar to modify the registration information and/or transfer the domain to another registrar, a form of identity theft. Once this has been done, the hijacker has full control of the domain and can use it or sell it to a third party. This can be financially devastating to the original domain name holder, who may have derived commercial income from a website hosted at the domain or conducted business through that domain's e-mail accounts. Additionally, the hijacker can use the domain name to facilitate illegal activity such as phishing, where a website is replaced by an identical website that records private information such as log-in passwords.

Responses to discovered hijackings vary; sometimes the registration information can be returned to its original state by the current registrar, but this may be more difficult if the domain name was transferred to another registrar, particularly if that registrar resides in another country. In some cases the original domain owner is not able to regain control over the domain.

The legal status of domain hijacking remains unclear. It is analogous with theft, in that the original owner is deprived of the benefits of the domain, but theft traditionally regards concrete goods such as jewelry and electronics, whereas domain name ownership is stored only in the digital state of the domain name registry, a network of computers. There are no specific laws regarding domain hijacking, nor any law that specifically holds the domain name registrar responsible for allowing the registrant information to be modified without the permission of the original registrant. In some cases there may be recourse under trademark law, but not all domain names are (or can be) registered as trademarks.