In March 2020, the Your COVID-19 Risk tool was developed in response to the global spread of SARS-CoV-2. The tool is an online resource based on key behavioural evidence-based risk factors related to contracting and spreading SARS-CoV-2. This article describes the development of the tool, the produced resources, the associated open repository, and initial results. This tool was developed by a multidisciplinary research team consisting of more than 150 international experts. This project leverages knowledge obtained in behavioural science, aiming to promote behaviour change by assessing risk and supporting individuals completing the assessment tool to protect themselves and others from infection. To enable iterative improvements of the tool, tool users can optionally answer questions about behavioural determinants. The data and results are openly shared to support governments and health agencies developing behaviour change interventions. Over 60 000 users in more than 150 countries have assessed their risk and provided data.