Purpose: Historically, graduate education's goal was to prepare academics; now most science, technology, engineering and/or mathematics (STEM) graduate students (GSs) go on to nonacademic careers. STEM GSs must be equipped for success regardless of career aspirations, which can be done by strengthening GSs' professional identities. This study aims to explore an interdisciplinary partnership designed to strengthen STEM GS professional identity. Design/methodology/approach: The STEM Partnership Project (SPP), asked STEM GSs to serve as disciplinary experts and teach STEM content to elementary teacher candidates (TCs) so the TCs could design and teach an elementary science lesson. GSs also enrolled in a one-credit course to support SPP participation and activities. Over five semesters, the authors collected data from 28 STEM GSs across different disciplines and degree programs in the form of course assignments, surveys and interviews. Findings: The SPP supported the development of a professional identity by having GSs serve as and feel like experts; increasing GSs' sense of belonging in their field; increasing GSs' self-confidence that they could (learn to) teach a wide variety of audiences; and raising GSs' awareness of their ability to serve others via their field. Originality/value: The SPP's outcomes were consistent across STEM disciplines, did not require GSs to take on large amounts of coursework, nor did it cost much beyond materials for the various lessons. Furthermore, the key components that strengthened GSs' professional identities could be adapted for different contexts and institutions.