Acceptance and Commitment Therapy (ACT) is an established psychological therapy, but its effectiveness for carers of people with multiple sclerosis (MS) experiencing carer-related strain has not been established. This study assessed the acceptability and feasibility of conducting a randomised controlled trial comparing ACT self-help, telephone-supported ACT self-help, and usual care. We describe a mixed-method, parallel three-armed feasibility randomised controlled trial. Participants were carers (i.e. caregivers) of people with MS. The self-help group received an ACT self-help text (covered over 8 weeks), the enhanced self-help group additionally received weekly telephone support. All participants completed questionnaires at baseline, 3-month, and 6-month post-randomisation, assessing carer strain, health-related quality of life, and ACT-related processes. A sample of participants was also interviewed. Twenty-four carers were randomised. Participants found the study procedures to be acceptable, but highlighted difficulties with the self-help text and timing of the intervention. An exploratory, group-level analysis indicated effectiveness for the enhanced self-help group on carer strain (consistent across both follow-ups), with convergent qualitative reports to support this. A full trial of ACT-based, telephone-supported self-help is warranted, including both the self-help and enhanced self-help design, following significant adaptions to the self-help itself. An internal pilot would, therefore, be recommended to further assess the feasibility after changes are incorporated.Trial registration: The trial was registered on ClinicalTrials.gov (NCT03077971).