IntroductionPatients with generalised anxiety disorder (GAD) have concerns and needs about their health and the healthcare they receive. Patient decision aids (PtDAs) are tools that assist patients in making health decisions, when there is uncertainty about treatment choice, incorporating their personal preferences and values about the available treatment options. PtDAs can improve shared decision-making and lead to better treatment outcomes. The aim of this study is to evaluate the effectiveness of a web-based PtDA for patients with GAD in primary care (PC).Methods and analysisThe general study design is comprised of two stages: (1) development of a web-based PtDA for patients with GAD, derived from an evidence-based Clinical Practice Guideline and (2) assessment of the effectiveness of the PtDA in a randomised controlled trial (RCT) design, in PC centres in Tenerife (Spain). This RCT will be carried out with 124 patients with GAD, comparing the PtDA to a fact sheet with general information on mental health. Patients will review the PtDA in one session accompanied by a researcher. Post-intervention measures will be administered immediately after the intervention and at 3-month follow-up. The primary outcome will be decisional conflict. Secondary outcomes will include knowledge about GAD and its treatment, treatment preference, concordance between treatment preference and choice, and decision quality (knowledge ≥60% and concordant decision).Ethics and disseminationThe project received ethics approval from the local committee at Nuestra Señora de la Candelaria (HUNSC) University Hospital in Santa Cruz de Tenerife (code: CHUNSC_2019_58). Informed consent will be obtained from each participant before randomisation. Results from the trial will be submitted for publication in international peer-reviewed scientific journals and will be disseminated through workshops and local and international conferences.Trial registration numberNCT04364958.