SummaryTwo out of three adults seeking treatment for alcohol or other substance use disorders report co‐occurring symptoms of insomnia. This study compared the feasibility, acceptability, and preliminary efficacy of cognitive behavioural therapy for insomnia (CBT‐I) among adults seeking and not seeking treatment for substance use. Adults with alcohol or other substance use disorders (n = 22, 32% female, 82% White; Mage = 39.5) completed assessments at baseline, post‐treatment, and at 6 week follow‐up. Of those, 11 were and 11 were not enrolled in substance use treatment. All received CBT‐I. Multiple imputation was used for missing data. Data were analysed using repeated measures analyses of variance. In the substance use treatment group, 6/11 completed post and 5/11 completed follow‐up. In the non‐treatment group, 9/11 completed post and 7/11 completed follow‐up. Participants in both groups reported improvements in insomnia severity, sleep onset latency, and dysfunctional beliefs about sleep, with most effects evident at post and follow‐up. There was a marginal group‐by‐time interaction in the change in frequency of substance use, with only participants not in substance use treatment reporting decreases at follow‐up. Participants in substance use treatment reported significant reductions in substance‐related problems and symptoms of post‐traumatic stress disorder over time; however, they also reported more symptoms at baseline. CBT‐I produces similar reductions in insomnia but is relatively less feasible among individuals in (versus not in) treatment for substance use disorder. This may be due to the more complex logistics of accessing CBT‐I among those in treatment. We speculate that integrating CBT‐I into treatment for addictions may improve feasibility in this population. clinicaltrials.gov NCT04198311.