Introduction: Regular physical activity (PA) is recommended by all type 2 diabetes mellitus (T2DM) management guidelines. The OPADIA study aimed to determine whether using a specific patient questionnaire (Optima-PA©) could help T2DM patients increase their PA by leading to better physician-patient communication and improved levels of shared decision making concerning Specific, Measurable, Acceptable, Realistic, Timely (SMART)-PA micro-objectives. Methods: Physicians participating in this multicentre, prospective, randomised, real-life study were allocated to a standard group (T2DM patients managed according to usual clinical practice, n = 24) or the OPTIMA-PA group (additional use of the questionnaire, n = 30). The main outcome was the percentage of inclusion visits ending with the setting up of at least one SMART-PA micro-objective. Other outcomes were the impact of the OPTIMA-PA questionnaire on patient perceptions of shared decision making (ENTRED questionnaire) and the impact of the OPTIMA-PA questionnaire and establishing SMART-PA micro-objectives as well as patient-perceived physician empathy (ENTRED questionnaire) and GP aptitude for patient-centredness (SEPCQ scores) on patient PA levels over a 3-month period (IPAQ-SF scores). Results: One hundred twenty-two patients were included in the standard group and 134 in the OPTIMA-PA group. Unexpectedly, more inclusion visits ended with SMART-PA micro-objectives being set up in the standard group (p < 0.001): 81.1% (n = 99/122) versus 59.7% (n = 80/134). However, fewer patients in the OPTIMA-PA group felt that GPs made decisions alone (32% versus 60%; p < 0.0001). Positive correlations were also observed between GP patient-centredness and patient-perceived GP empathy or increased patient PA over the study period. Conclusion: Although the OPTIMA-PA questionnaire did not directly promote setting up of SMART-PA micro-objectives in T2DM patients, the OPADIA study demonstrated that this tool was effective at improving patient-physician relationships by increasing patient involvement in therapeutic decision making. Our study also highlighted the importance of GP aptitude for patient-centredness for improving PA in T2DM patients.