Background Web-based tools for people with dementia and their family caregivers have considerably increased over the years and offer promising solutions to several unmet needs such as supporting self-care in daily life, facilitating treatment delivery, or ensuring their ability to communicate. The use of web-based tools in the field of advance care planning (ACP) for people with dementia and their family caregivers has yet to be explored and requires careful consideration, given the sensitive topic and the specific needs of people with dementia and their families. Objective This paper reports the protocol for a study aiming to develop and simultaneously test the usability of an ACP website designed for, and with, people with dementia and their families. Methods The development of the website is based on a process map for the development of web-based decision support interventions and on the Medical Research Council framework for complex intervention development and evaluation. Additionally, we apply a user-centered approach in combination with patient and public involvement (PPI) throughout the development process. We describe our iterative development approach to the website. Participants and a PPI group give feedback on 4 prototypes of the ACP website. For each iteration, we aim to include 12 participants (3 people with dementia, 3 family caregivers, and 3 dyads) in usability testing. In the first 3 iterations, usability testing includes (1) a think-aloud exercise, (2) researcher observations, and (3) the System Usability Scale questionnaire. The last iteration of usability testing is composed of a semistructured interview assessing the layout, content, face validity, and readability of the website. Qualitative data from the think-aloud exercises and interviews are analyzed using thematic analysis. Mean scores are calculated for the System Usability Scale questionnaire. Results This study received approval from the Ethical Review Board of Brussels University Hospital of the Vrije Universiteit Brussel. Recruitment began in October 2021. The target date for paper submission of the results of the development and usability testing will be in 2023. Conclusions The methods in this protocol describe a feasible and inclusive approach to the development of an ACP website together with people with dementia, their family caregivers, and other stakeholders. We provide a clear overview of how to combine PPI input and user-centered development methods, leading to a transparent and reliable development process. This protocol might stimulate the active participation of people with dementia, their caregivers, and regional stakeholders in future studies on web-based technologies. The results of this study will be used to refine the design and create a relevant and user-friendly ACP website that is ready to be tested in a larger evaluation study. International Registered Report Identifier (IRRID) DERR1-10.2196/46935