Objective: Clinicians are tasked with assessing the relationship between violence and mental illness. Yet, there is now a legal expectation in some countries that public services, including health professionals, assess risk of violent extremism – with few available measures. We previously developed a new measure of Sympathies for Radicalisation, using items that measure Sympathies for Violent Protest and Terrorism. In this paper, we present the detailed psychometric properties of a reduced item measure of Sympathies for Violent Protest and Terrorism. We use data from two studies to test replication and then validity against outcomes of self-reported violence and convictions in the entire sample and in those with depressive symptoms. Methods: Data from two cross-sectional neighbourhood surveys, consisting of Pakistani and Bangladeshi adults (survey 1, n = 608) and White British and Pakistani adults (survey 2, n = 618), were used to undertake confirmatory factor analysis of Sympathies for Radicalisation and produce a short measure of Sympathies for Violent Protest and Terrorism. Survey 2 data were used to test the Sympathies for Violent Protest and Terrorism’s convergent validity to classify presence/absence of violence and convictions in the whole sample and for subgroups by depressive disorder. Results: The seven-item measure’s structure was a consistent measure of extremist attitudes across the two surveys. A threshold score of zero to classify violence was optimal (specificity = 89.7%; area under the curve = 0.75), but sensitivity to a risk of violence was poor (34.5%). The short version Sympathies for Radicalisation was a better classifier of violence in respondents with depression, dysthymia or both (area under the curve = 0.78) than respondents with neither (area under the curve = 0.69; β = 0.62, 95% confidence interval = [−0.67, 1.92]; standard error = 0.66). Conclusion: The seven-item measure of Sympathies for Violent Protest and Terrorism is an accessible and valid measure for clinical assessments and helpfully identifies low risk of violence. It enables clinicians to conduct detailed assessments of people endorsing one or more of the items, although further research is needed.