Background Web-based tools (e.g., websites, apps) for people with dementia and their family caregivers may be useful in supporting advance care planning (ACP). Using a user-centred design approach, we developed an ACP website for people with dementia and their families. This protocol describes how we will test and evaluate the ACP website. Publishing a study protocol can guide others who want to evaluate web-based tools. Moreover, the data collection methods used in this study are very innovative since they aim to involve people living with dementia without overburdening them. Methods We will conduct an evaluation study of the ACP website in Flanders, Belgium, using a convergent parallel mixed methods pre-post-test design with continuous follow-up. Thirty eligible dyads of people with mild to moderate dementia (both early and late onset) and their family caregivers will use the website in their everyday life for 8 weeks. We will evaluate the usage, usability, acceptability, and feasibility of the website, as well as the experiences of users. Additionally, we evaluate the effects of using the website on ACP readiness, ACP knowledge, attitudes, perceived barriers to engage in ACP, self-efficacy and skills to engage in ACP Results Recruitment and data collection is foreseen between end of 2022 and 2023. Conclusion This evaluation study of an ACP website for people with dementia and their family caregivers will be the first to evaluate how a web-based tool can support people living with dementia and their families in ACP. The strength of this study lies in the combination of interviews, surveys, and ongoing data logging, which provide insights into the use of support tools in people's daily context. We expect that recruiting people with dementia and their families will be difficult so we have set up a thorough strategy to reach the anticipated sample size.