In a world of burgeoning societal issues, future scientists must be equipped to work interdisciplinarily to address real-world problems. To train undergraduate students toward this end, practitioners must also have quality assessment tools to measure students' ability to think within an interdisciplinary system. There is, however, a dearth of instruments that accurately measure this competency. Using a theoretically and empirically based model, we developed an instrument, the Interdisciplinary Science Rubric (IDSR), to measure undergraduate students' interdisciplinary science thinking. An essay assignment was administered to 102 students across five courses at three different institutions. Students' work was scored with the newly developed rubric. Evidence of construct validity was established through novice and expert response processes via semistructured, think-aloud interviews with 29 students and four instructors to ensure the constructs and criteria within the instrument were operating as intended. Interrater reliability of essay scores was collected with the instructors of record ([kappa] = 0.67). An expert panel of discipline-based education researchers (n = 11) were consulted to further refine the scoring metric of the rubric. Results indicate that the IDSR produces valid data to measure undergraduate students' ability to think interdisciplinarily in science.