Background Family members of intensive care unit (ICU) patients are at risk for post–intensive care syndrome– family (PICS-F), including symptoms of anxiety, depression, and posttraumatic stress. Cognitive behavioral therapy is the first-line nonpharmacologic treatment for many psychological symptoms and has been successfully delivered by use of mobile technology for symptom self-management. Objectives To determine the feasibility of delivering cognitive behavioral therapy through a smartphone app to family members of critically ill patients. Methods This was a prospective longitudinal cohort study with a consecutive sample of patients admitted to 2 adult ICUs and their family members. The control group period was followed by the intervention group period. The intervention consisted of a mobile health app preloaded on a smartphone provided to family members. The study time points were enrollment (within 5 days of ICU admission), 30 days after admission, and 60 days after admission. Study measures included demographic data, app use, satisfaction with the app, mental health self-efficacy, and measures of PICS-F symptoms. Results The study sample consisted of 49 predominantly White (92%) and female (82%) family members (24 intervention, 25 control). Smartphone ownership was 88%. Completion rates for study measures were 92% in the control group and 79% in the intervention group. Family members logged in to the app a mean of 18.58 times (range 2-89) and spent a mean of 81.29 minutes (range 4.93-426.63 minutes) using the app. Conclusions The study results confirm the feasibility of implementing app-based delivery of cognitive behavioral therapy to family members of ICU patients.