The objective of the research was to evaluate the effectiveness of ACT in optimizing adherence to ART. A quantitative study was conducted with a Solomon experimental design and follow-up. The sample comprised 84 seropositive patients randomly selected and distributed into four groups, two experimental and two controls. Symptoms of anxiety and depression (Hospital Anxiety and Depression Scale), level of treatment adherence (Questionnaire for Adherence to Treatment for HIV / AIDS) and experiential avoidance (Acceptance and Action Questionnaire) were measured. An ACT-based group intervention protocol was applied to the experimental groups, consisting of eight sessions. The results showed negative correlation between adherence and experiential avoidance, anxiety and depression. They also showed positive effects of ACT-based intervention on adherence to ART, with increase in adherence in the experimental groups and statistical difference in adherence in comparison with the control groups, according to the ANOVA test. It is concluded conclude that ACT is effective in optimising adherence to ART, allowing for behavioural changes that promote adaptation to diagnosis and treatment and reducing the presence of avoidance behaviours and emotional involvement.