Background: Given the degenerative nature of the condition, people living with motor neuron disease experience high levels of psychological distress. The purpose of this research was to investigate the cost-effectiveness of Acceptance and Commitment Therapy (ACT), adapted for the specific needs of this population, for improving quality of life. Methods: A trial-based cost-utility analysis over a 9-month period was conducted comparing ACT plus usual care (n = 97) versus usual care alone (n = 94) from the perspective of the National Health Service. In the primary analysis, quality-adjusted life years (QALYs) were computed using health utilities generated from EQ-5D-5L. Sensitivity analyses and subgroup analyses were also carried out. Results: Difference in costs was statistically significant between the two arms, driven mainly by the intervention costs. Effects measured by EQ-5D-5L were not statistically significantly different between the two arms. The incremental cost-effectiveness was above the £20,000 to £30,000 per QALY gained threshold used in the United Kingdom. However, the difference in effects was statistically significant when measured by the McGill Quality of Life-Revised questionnaire (MQOL-R). The intervention was cost-effective in a subgroup experiencing medium deterioration in motor neuron symptoms. Conclusions: Despite the intervention being cost-ineffective in the primary analysis, the significant difference in the effects measured by MQOL-R, the low costs of the intervention, the results in the subgroup analysis and the fact that ACT was shown to improve the quality of life for people living with MND, suggest that ACT could be incorporated into MND clinical services