Religious radicalism has become an issue in Indonesia due to its link to terrorism acts. The government raised the need to conduct an assessment on radicalism. This study aims to develop the Religious Radicalism Scale (ReadS). The study was arranged into four general phases: 1) Defining religious radicalism, 2) Identifying indicators of religious radicalism, 3) Developing scale, and 4) Identifying psychometric properties. The study applied quantitative method of analysis to examine the instrument’s reliability (alpha Cronbach and omega McDonnald) and content-related validity. It involved 31 multidisciplinary scholars and professionals in identifying indicators of religious radicalism, followed by 64 participants from two contrasted groups to define psychometric properties. The findings demonstrate the efficacy of ReadS’ items in detecting religious radicalism. Moreover, ReadS achieves satisfactory reliability α=0.97 and ω=0.97; hence, the instrument can be applied in investigating religious radicalism. The study may help academics and practitioners in scientifically examining religious radicalism.