Operations security (OPSEC) is a process that identifies critical information to determine if friendly actions can be observed by enemy intelligence, determines if information obtained by adversaries could be interpreted to be useful to them, and then executes selected measures that eliminate or reduce adversary exploitation of friendly critical information.

In a more general sense, OPSEC is the process of protecting individual pieces of data that could be grouped together to give the bigger picture (called aggregation). OPSEC is the protection of critical information deemed mission-essential from military commanders, senior leaders, management or other decision-making bodies. The process results in the development of countermeasures, which include technical and non-technical measures such as the use of email encryption software, taking precautions against eavesdropping, paying close attention to a picture you have taken (such as items in the background), or not talking openly on social media sites about information on the unit, activity or organization's Critical Information List.

The term "operations security" was coined by the United States military during the Vietnam War.