A security guard (also known as a security officer, security inspector, factory guard, or protective agent) is a person employed by an organization or individual to protect their assets from a variety of hazards (e.g. crime, waste, damage, unsafe behavior, etc.) by enforcing preventative measures. These assets may include property, people, equipment, money, housing societies, etc. Guards protect them by maintaining a high-visibility presence to deter illegal and inappropriate actions. They may do that by looking (either directly through patrols or indirectly by monitoring alarm systems or video surveillance cameras) for signs of crime or other hazards and taking action to minimize the damage. The actions can include warning and escorting trespassers off property, reporting incidents to clients, reporting to emergency services, and informing police or emergency medical services as appropriate.

Security guards are generally uniformed in order to represent their lawful authority to protect private property. They are generally governed by legal regulations that set out eligibility requirements, such as a criminal record check, fitness levels, and permits or licenses needed from local authorities. Several countries prescribe a preference in hiring veterans or people with experience in police or paramilitary services. The authorities permitted for security guards vary by country and sub-national jurisdiction. They are hired by a range of organizations, including businesses, government departments and agencies, and not-for-profit organizations.

Until the 1980s, the term watchman was more commonly used for this function, a usage dating back at least to the Middle Ages. This term was carried over to North America, where it was interchangeable with night watchman until modern security-based titles replaced both terms. Security guards are sometimes regarded as fulfilling the function of private policing.