Educators are often trying to foster Science, Technology, Engineering, and Math (STEM) engagement by facilitating interest in STEM content, but research suggests that STEM interest generally declines through the primary and secondary grades. To counter this trend, STEM interventions have begun to focus on younger students. To support this work, we developed the Elementary Activity Interest Measure (EAIM) to allow for teachers and researchers to more easily measure interest in younger populations. Embedded within a large, randomized control trial intervention designed to increase learning about STEM content, our data from both a pilot study (16 classrooms comprised of 194 students) and the RCT (all fourth-grade classrooms in a large, urban school district comprised of 1712 students) indicate that the proposed use of the EAIM scores are supported by the collected evidence of reliability and validity. Further, EAIM scores correlate both with performance in science (Polikoff et al., 2018) as well as positive and negative emotions. Utilizing graphics and targeted specifically for elementary aged students, the EAIM is designed to measure interest in large-scale studies and is easily modifiable for teachers/practitioners to check if their interventions are producing the desired results.