Although family members of cancer patients are at great risk of experiencing psychological distress, clinical tools to assist with recognizing and intervening with appropriate psychosocial care are sparse. This study reports on the first validation of the distress thermometer (DT) as a screening instrument for symptoms of depression and anxiety in family members of cancer patients. The DT was administered with the Hospital Anxiety and Depression Scale (HADS) in a sample of 321 family members. Receiver operating characteristics (ROC) demonstrated that the DT has good diagnostic utility relative to the HADS (area under the curve= 0.88 relative to the HADS anxiety scale; 0.84 relative to the HADS depression scale, respectively). The ROC curves indicate that using a cut-off of 4/5 maximizes sensitivity (86.2% HADS anxiety scale; 88.2% HADS depression scale) and specificity (71.2% HADS anxiety scale; 67.6% HADS depression scale); however, the alternative lower cut-off of 3/4 increases sensitivity (94.1% for both scales) and hence reduces the risk of missing distressed family members (specificity is 62.9% for HADS anxiety scale; 59.1% for HADS depression scale). The results offer validation of the DT for screening family members of cancer patients and support its use for clinical assessment. Distress screening with DT for family members of cancer patients is a promising and efficient approach to integrating family members in the program of care and provides the first step toward meeting their unmet needs with referral for supportive services.