A security guard (also known as a security inspector, security officer, factory guard, or protective agent) is a person employed by an organisation or individual to protect their employer'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 subnational jurisdiction. Security officers are hired by a range of organizations, including businesses, government departments and agencies and not-for-profit organizations (e.g., churches and charitable organizations).

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

Many security firms and proprietary security departments practice the "detect, deter, observe and report" method. Security officers are usually not required to make arrests, but have the authority to make a citizen's arrest and detain them, or otherwise act as an agent of law enforcement, for example, at the request of a police officer or a sheriff. Some security officers may also be armed, with functions similar to an auxiliary police.

A private security officer's responsibility is protecting their client from a variety of hazards (usually in the form of criminal acts). Security personnel enforce company rules and can act to protect lives and property, and they sometimes have a contractual obligation to provide these actions. In addition to basic deterrence, security officers are often trained to perform specialized tasks such as arrest and control (including handcuffing and restraints), operate emergency equipment, perform first aid, CPR, take accurate notes, write detailed reports, and perform other tasks as required by the client they are serving. All security officers are also required to go through additional training mandated by the state for the carrying of weapons such as batons, firearms, and pepper spray (e.g. the Bureau of Security and Investigative Services in California has requirements that a license for each item listed must be carried while on duty). Some officers are required to complete police certification for special duties. In recent years, due to elevated threats of terrorism, most security officers are required to have bomb-threat training and/or emergency crisis training, especially those located in soft target areas such as shopping malls, schools, and any other area where the general public congregate. One major economic justification for security personnel is that insurance companies (particularly fire insurance carriers) will give substantial rate discounts to sites which have a 24-hour presence. For a high risk or high-value property, the discount can often exceed the money being spent on its security program. Discounts are offered because having security on site increases the odds that any fire will be noticed and reported to the local fire department before a total loss occurs. Also, the presence of security officers (particularly in combination with effective security procedures) tends to diminish "shrinkage", theft, employee misconduct, and safety rule violations, property damage, or even sabotage. Many casinos hire security officers to protect money when transferring it from the casino to the casino's bank.

Security personnel may also perform access control at building entrances and vehicle gates; meaning, they ensure that employees and visitors display proper passes or identification before entering the facility. Security officers are called upon to respond to potential hazards (such as broken lights or doors, disturbances, lost persons, minor injuries, etc.) and to assist in serious emergencies (medicals, fires, crime, etc.) by securing the scene to prevent further loss or damage, summoning emergency responders to the incident, helping to redirect foot traffic to safe locations, and by documenting what happened on an incident report to give their client an idea of how to prevent similar situations from occurring. Armed security officers are frequently contracted to respond as law enforcement until a given situation at a client location is under control and/or public authorities arrive on the scene.

Patrolling is usually a large part of a security officer's duties, as most incidents are prevented by being looked for instead of waiting for them to occur. Often these patrols are logged by use of a guard tour patrol system, which require regular patrols. Until recently the most commonly used form used to be mechanical clock systems that required a key for manual punching of a number to a strip of paper inside with the time pre-printed on it. But recently, electronic systems have risen in popularity due to their lightweight, ease of use, and downloadable logging capabilities. Regular patrols are, however, becoming less accepted as an industry standard, as it provides predictability for the would-be criminal, as well as monotony for the security officer on duty. Random patrols are easily programmed into electronic systems, allowing greater freedom of movement and unpredictability. Global positioning systems are beginning to be used because they are a more effective means of tracking officers' movements and behavior.

Although security officers differ from police officers, military personnel, federal agents/officers, and the like, Australia and the United States have a growing proportion of security personnel that has former police or military experience, including senior management personnel.^{[citation needed]} On the other hand, some security officers, young people, in particular, use the job as a practical experience to use in applying to law enforcement agencies.^{[citation needed]}