While Acceptance and Commitment Therapy for Insomnia (ACT-I) has been proposed as a promising alternative to Cognitive Behavioral Therapy for Insomnia, its efficacy as a distinct alternative, without sleep restriction and stimulus control, remains largely unknown. In this protocol paper, we describe a randomized controlled trial that aims to test the efficacy of ACT-I as a stand-alone intervention for insomnia. Adults with insomnia (N = 80) will be randomly allocated to five individual sessions of ACT-I or a waitlist control group. The main objective is to assess whether ACT-I is superior to the control group in improving insomnia severity, alongside secondary outcomes including sleep diary measures, anxiety, depression, general well-being, and sleep-related quality of life. Additionally, we aim to explore potential mechanisms of ACT-I, including psychological (in)flexibility, sleep-related arousal, dysfunctional cognitions, and sleep-related safety behaviors. Both the treatment and waiting period span 7 weeks. Assessments take place at baseline (pre), after 4 weeks (mid), and after 8 weeks (post), followed by a 3- and 6-month follow-up for the ACT-I group. Treatment effects will be analyzed with mixed linear regression based on the intention-to-treat principle, and potential mechanisms will be explored with network intervention analysis. This study contributes to the understanding of ACT-I’s treatment effects and potential working mechanisms, informing clinical practice on whether ACT-I without sleep restriction or stimulus control could provide an adequate alternative treatment for insomnia. Trial registration number: NCT06336551.