A trauma trigger is a psychological stimulus that prompts involuntary recall of a previous traumatic experience. The stimulus itself need not be frightening or traumatic and may be only indirectly or superficially reminiscent of an earlier traumatic incident, such as a scent or a piece of clothing. Triggers can be subtle, individual, and difficult for others to predict. A trauma trigger may also be called a trauma stimulus, a trauma stressor or a trauma reminder.

The process of connecting a traumatic experience to a trauma trigger is called traumatic coupling. When trauma is "triggered", the involuntary response goes far beyond feeling uncomfortable and can feel overwhelming and uncontrollable, such as a panic attack, a flashback, or a strong impulse to flee to a safe place. Avoiding a trauma trigger, and therefore the potentially extreme reaction it provokes, is a common behavioral symptom of posttraumatic stress disorder (PTSD) and post-traumatic embitterment disorder (PTED), a treatable and usually temporary condition in which people sometimes experience overwhelming emotional or physical symptoms when something reminds them of, or "triggers" the memory of, a traumatic event. Long-term avoidance of triggers increases the likelihood that the affected person will develop a disabling level of PTSD. Identifying and addressing trauma triggers is an important part of treating PTSD.

A trigger warning is a message presented to an audience about the contents of a piece of media, to warn them that it contains potentially distressing content. A more generic term, which is not directly focused on PTSD, is content warning.

The trigger can be anything that provokes fear or distressing memories in the affected person, and which the affected person associates with a previous traumatic experience. Just as trauma is not merely an unpleasant or adverse experience, a trauma trigger is not merely something that makes a person feel uncomfortable or offended. Some common triggers are:

The trigger is usually personal and specific. However, it need not be closely related to the actual experience. For example, after the Gulf War, some Israelis experienced the sound of an accelerating motorbike as a trigger, which they associated with the sound of sirens they heard during the war, even though the resemblance between the two sounds is limited.

The realistic portrayal of graphic violence in visual media may expose some affected people to triggers while watching movies or television.

People who have experienced trauma and who have developed trauma triggers may panic when the trigger is experienced, especially if it is unexpected. For example, the noise of fireworks may seem unbearable to a combat veteran whose trauma is coupled with sudden, loud noises as the trigger.

Trigger warnings, sometimes called content warnings, are warnings that a work contains writing, images, or concepts that may be distressing to some people. Content warnings have been widely used in mass media without any connection to trauma, such as the US TV Parental Guidelines, which indicate that a show includes content that some families may find inappropriate for their children. The term trigger warning, with its trauma-specific context, originated at feminist websites that were discussing violence against women, and then spread to other areas, such as print media and university courses. Although it is widely recognized that any sight, sound, smell, taste, touch, feeling or sensation could be a trigger, trigger warnings are most commonly presented on a relatively narrow range of material, especially content about sexual abuse and mental illness (such as suicide, eating disorders, and self-injury).