Insomnia is highly prevalent in the adolescent population and frequently occurs in the context of other medical or mental health concerns. Efficacy of cognitive–behavioral therapy for insomnia (CBT-I) has been determined in adults with comorbid conditions. However, there are limited data applying CBT-I to adolescents with comorbid conditions. Therefore, the purpose of this study was to (a) develop and refine a 4-session CBT-I intervention for adolescents with co-occurring medical and mental health conditions, and (b) evaluate feasibility and acceptability of applying the intervention to adolescents and their parents. Forty participants (ages 11 to 18 years) were recruited from 2 pediatric specialty clinics representing a range of physical and psychiatric comorbidities (e.g., depression, chronic pain, anxiety). Adolescents and parents attended 4 treatment sessions of CBT-I delivered individually or conjointly to adolescent and parent. Daily sleep diaries were completed during the treatment period. Preliminary findings demonstrated a high level of feasibility and acceptability of treatment. Compliance with treatment visits was high, with 34 of the 40 families (85%) completing all 4 sessions. Youth and parents were highly engaged in therapy sessions as rated by treating therapists. On the Treatment Evaluation Inventory, parents’ and teens’ mean scores indicated high treatment acceptability (M = 38.8, SD = 5.2; M = 36.8, SD = 4.6, respectively). The preliminary data suggest that CBT-I is feasible to implement to treat insomnia in adolescents with co-occurring health and mental health conditions. Future studies are needed to evaluate intervention efficacy on sleep and functional outcomes in youth.