Objective: To evaluate the impact of an Acceptance and Commitment Therapy (ACT) programme, tailored for people living with type 1 diabetes, on glycated haemoglobin (HbA1c), self-management and psychosocial factors among individuals with HbA1c>60 mmol/mol compared with treatment as usual (TAU). Setting: An endocrinologic clinic in Sweden. Participants: In this randomised controlled trial, 81 individuals with type 1 diabetes, aged 18–70 years with HbA1c>60 mmol/mol, were randomly assigned to either an ACT group intervention or TAU. Exclusion criteria were: unable to speak Swedish, untreated or severe psychiatric disease, cortisone treatment, untreated thyroid disease and newly started insulin pump therapy. At the 2-year follow-up, HbA1c was measured in 26 individuals. Intervention: The ACT programme comprised seven 2-hour sessions held over 14 weeks and focused on acceptance of stressful thoughts and emotions, and to promote value-based committed action. Outcomes: The primary outcome was HbA1c, and the secondary outcomes were measures of depression, anxiety, general stress, fear of hypoglycaemia, diabetes distress, self-care activities, psychological flexibility (general and related to diabetes) and quality of life. The primary endpoint was HbA1c 2 years after the intervention programme. Linear mixed models were used to test for an interaction effect between measurement time and group. Results: Likelihood ratio test of nested models demonstrated no statistically significant interaction effect (χ2=0.49, p=0.485) between measurement time and group regarding HbA1c. However, a statistically significant interaction effect (likelihood ratio test χ2=12.63, p<0.001) was observed with improved scores on The Acceptance and Action Questionnaire in the intervention group after 1 and 2 years. Conclusions: No statistically significant difference was found between the groups regarding the primary outcome measure, HbA1c. However, the ACT programme showed a persistent beneficial impact on psychological flexibility in the intervention group. The dropout rate was higher than expected, which may indicate a challenge in this type of study. Trial registration number NCT02914496.