A Security guard (also known as a security inspector, security officer, or protective agent) is a person employed by a government or private party to protect the employing party’s assets (property, people, equipment, money, etc.) from a variety of hazards (such as crime, waste, damages, unsafe worker behavior, etc.) by enforcing preventative measures. Security guards do this by maintaining a high-visibility presence to deter illegal and inappropriate actions, looking (either directly through patrols, or indirectly by monitoring alarm systems or video surveillance cameras) for signs of crime or other hazards (such as a fire), taking action to minimize damage (such as warning and escorting trespassers off property), and reporting any incidents to their clients and emergency services (such as the police or emergency medical services), as appropriate.

Security officers are generally uniformed to represent their lawful authority to protect private property. Security guards are generally governed by legal regulations, which set out the requirements for eligibility (such as a criminal record check) and the permitted authorities of a security guard in a given jurisdiction. The authorities permitted to security guards vary by country and sub national jurisdiction. Security officers are hired by a range of organizations, including businesses, government departments and agencies and not-for-profit organizations.

This term was carried over to Canada where it was interchangeable with night-watchman (e.g. security guard) until both terms were replaced with the modern security-based titles. Security officers are sometimes regarded as fulfilling a private policing function.