Environmental psychology heavily relies on psychometric scales to approach relevant psychological constructs. Traditionally, these scales have most often been developed using classical test theory, despite the availability of more advanced methods like Item Response Theory (IRT)—a specific form of “modern test theory”. The increasing capabilities of statistical software and the growing availability of open-source tools such as R packages have made IRT analyses more accessible and easier to implement. Adopting such approach would significantly benefit the field by enhancing the rigour and precision of our measurement instruments. In this short note, we present a practical example of applying IRT to developing a short scale of environmental risk perception (assessing perceived likelihood, seriousness, and concern about threats related to biodiversity loss and climate change). We use data from a large-scale survey of the views of the population of Finland about biodiversity and other environment-related issues (N = 2005). In a dual-step process of confirmatory factor analysis followed by IRT, we demonstrate evidence of validity and reliability of the 6-item environmental risk perception scale in the context of a national study. We illustrate how IRT offers a more informative and comprehensive evaluation of specific items (assessing their location, discrimination, and information) and, therefore, of the overall scale (information and conditional reliability) compared to classical test theory. We advocate for the broader adoption of IRT within environmental psychology to improve the quality of the instruments we rely upon as a field.