CARE Plus is a primary-care-based complex intervention for patients with multimorbidity living in areas of high socioeconomic deprivation. This study explores patients' experience of the intervention and whether self-determination theory is useful to understand reported impacts. Thematic analysis of semistructured interviews of 14 participants conducted during a randomised controlled trial of CARE Plus. Improvement in wellbeing in daily lives following CARE Plus was estimated from participants' accounts of their experiences of the intervention. Participants valued the CARE Plus consultations irrespective of perceived improvements. Six participants reported changes in wellbeing that improved daily life, three reported slight improvement (not impacting daily life) and five no improvement. Evidence of satisfaction of the three major self-determination theory psychological needs - relatedness, competence and autonomy - was prominent in the accounts of those experiencing improved wellbeing in daily life; this group also spoke in ways congruent with more self-determined motivational regulation. These changes were not evident in those with little or no improvement in wellbeing. This study suggests self-determination theory has utility in understanding the impact of CARE Plus on patients and may be a useful theory to inform development of future interventions to improve outcomes for patients with multimorbidity.