Behavioral scales allow for the pain assessment of vulnerable critically ill patients who are unable to self-report. However, validity of the use of such scales is limited in traumatic brain injury patients with an altered level of consciousness as a result of the different way that these patients express pain. Family participation is considered as an important component of pain assessment for those unable to self-report, but research in this area is minimal so far. This study aimed to describe what behaviors family caregivers deemed relevant to pain for patients with a traumatic brain injury with an altered level of consciousness in the intensive care unit. Using a mixed-method descriptive design, semistructured interviews were conducted and behaviors' relevance was quantitatively rated by seven family caregivers of nonverbal patients with traumatic brain injury in the intensive care unit of a tertiary trauma center in Montreal, Canada. Family caregivers were able to provide rich descriptions of a number of behaviors they observed in their loved ones that were perceived to be relevant indicators of pain, such as muscle tension and key facial expressions and body movements. Several factors influenced how behaviors were interpreted by family, including personal medical beliefs and intimate knowledge of the patient's history. The pain behaviors determined by family caregivers can be useful in the pain assessment process of traumatic brain injury patients with an altered level of consciousness. Their input could also be helpful in further development of pain assessment tools.