This article describes the development, validation and application of a Rasch-based instrument, the Elementary School Science Classroom Environment Scale (ESSCES), for measuring students' perceptions of constructivist practices within the elementary science classroom. The instrument, designed to complement the Reformed Teaching Observation Protocol (RTOP), is conceptualised using the RTOP's three construct domains: Lesson Design and Implementation; Content; and Classroom Culture. Data from 895 elementary students was used to develop the Rasch scale, which was assessed for item fit, invariance and dimensionality. Overall, the data conformed to the assumptions of the Rasch model. In addition, the structural relationships among the retained items of the Rasch model supported and validated the instrument for measuring the reformed science classroom environment theoretical construct. The application of the ESSCES in a research study involving fourth grade students provides evidence that educators and researchers have a reliable instrument for understanding the elementary science classroom environment through the lens of the students.