In computing, a Trojan horse (also written as trojan) is malicious software that misleads users of its true purpose. Sometimes a Trojan horse does exactly what it claims to do but it also does something else. This is because its real hidden purpose is to perform malicious actions in the background, such as allow a stranger to read and change the computer's information. In some cases the user notices, in other cases they do not. Although a Trojan horse can contain any type of harmful code, many modern forms act as a backdoor, which bypasses normal authentication or encryption in a computer.

Trojans are generally spread by some form of trickery, for example getting users to click on bad popups, Email, text, attachments, advertisements, or fake device driver updates. A trojan may act as a "backdoor" to the computer, contacting a controller which can then have unauthorized access to the affected computer. Trojans may also access users' personal information such as banking information, passwords, or personal identity. It can infect other devices connected to their networks.

It is rare to get a trojan on your computer by merely visiting a website, though it can happen. Usually, though, trojans are attached to something the user downloads, such as music, wallpapers, backgrounds, games, and especially programs (apps).

Trojans were named after the Trojan Horse in Greek mythology, which was a large wooden horse, given by the Trojans (from Troy) to the Greeks (from Greece) as a gift, but when the Greeks brought it inside their city walls, Trojan soldiers came out and defeated the city.