The term homeland security refers to a security effort by a government to protect a nation against perceived external or internal threat.

The term is almost exclusively used in the United States; elsewhere, the activities of "homeland security" fall under a combination of national security and associated security services or the customs services of the country (for example, HM Revenue and Customs in the UK). The term arose following a merger of several U.S. governmental agencies to form the United States Department of Homeland Security after the September 11 attacks, and may be used to refer to the actions of that department.