Objective: Statin choice, an encounter decision aid (DA), was developed in the USA to facilitate shared decision making between patients and clinicians about the use of statins to reduce cardiovascular risk. We aimed to assess the efficacy of this DA, compared to usual primary care, in Spanish patients with type 2 diabetes. Methods: Cluster randomized trial with 29 clinicians and 168 patients. Knowledge of statins, cardiovascular risk perception, decisional conflict, anxiety and satisfaction with the decision making process were assessed immediately after intervention, and self-reported adherence at three months. Results: Intervention significantly improved knowledge (p = 0.01), perception of the 10-year risk of myocardial infarction without (p = 0.01) and with statins (p = 0.08), and satisfaction (p = 0.01). There were no significant differences in decisional conflict, anxiety, consultation time or adherence, although more intervention patients reported taking all pills during the last week (92.7% vs. 81%; p = 0.19). Conclusion: The statin choice DA improved the quality of decision making about statins. Practice implications: This trial contributes to the body of evidence substantiating the efficacy of statin choice and extending it to Spanish clinicians and their patients with type 2 diabetes. Trial registration: This trial is registered with the European Union Clinical Trials Register (EudraCT: 2010-023912-14).