<b><i>Objective:</i></b> Endometriosis is a chronic gynaecologic disease, causing pain and infertility. As there is no definitive cure, patients are subjected to long-term care. This study aimed to improve patient-centred endometriosis care. Patient-centredness of endometriosis care can be evaluated using the validated ENDOCARE questionnaire (ECQ), resulting in centre-specific targets for improvement. To understand how to tackle the targets for improvement as found by the ECQ, focus groups can be organized. <b><i>Design:</i></b> This protocol presents a prospective study with a mixed-methods approach to improve patient-centredness of endometriosis care. The study consists of 5 steps: (1) evaluating current patient-centredness of endometriosis care by using the ECQ, (2) understanding targets for improvement, (3) drafting an improvement plan, (4) implementing improvements, and (5) evaluating the improved patient-centredness of endometriosis care. The final evaluation will be performed 1.5 years after implementing the improvement plan. <b><i>Methods:</i></b> Patient-centredness will be evaluated using the ECQ by inviting women with endometriosis to participate (steps 1 and 5). To investigate step 2, focus groups will be organized. For these focus groups, women with endometriosis are asked to participate until data saturation is achieved. During focus groups, participants are motivated to discuss the found targets for improvement and stimulated to find ways to improve them. The drafting and implementing of the improvement plan (steps 3 and 4) will be organized with the help of health-care providers in close collaboration with the patient organization. To assess whether the implementation of the improvement plan was successful in improving endometriosis care, the results from the ECQ in step 5 will be compared to the results from the ECQ in step 1. Ethical approval was granted by the local Institutional Review Board (Ref 2018.438). <b><i>Setting:</i></b> The study was conducted in the university hospital in the Netherlands. <b><i>Limitations:</i></b> Both patients and health-care providers will be involved in drafting the improvement plan. By making the health-care providers responsible for improving care, the chance of succeeding is optimized. Whether this improvement strategy is successful will be investigated after the implementation of the improvement plan. The improvement plan is clinic specific and can possibly not be extrapolated to other endometriosis clinics. In order to aim to improve patient-centred endometriosis care elsewhere, the complete study protocol should be performed. <b><i>Conclusions:</i></b> This study protocol aimed to investigate focus groups as a strategy to identify possible interventions to improve patient-centred endometriosis care by investigating the underlying causes for poor performance on patient-centred care. This study protocol could be used in more endometriosis care centres in the future and might also be useful for improving patient-centredness in other chronic diseases.