A security alarm is a system designed to detect intrusion, such as unauthorized entry, into a building or other areas such as a home or school.

Security alarms used in residential, commercial, industrial, and military properties protect against burglary (theft) or property damage, as well as personal protection against intruders.

Some alarm systems serve a single purpose of burglary protection; combination systems provide fire and intrusion protection. Intrusion-alarm systems are combined with closed-circuit television surveillance (CCTV) systems to record intruders' activities and interface to access control systems for electrically locked doors.