Transdisciplinary research is increasingly important for solving complex societal problems. Institutions aim to train graduate students to engage in transdisciplinary research. However, limited evidence exists regarding the process by which students learn transdisciplinary thinking. This qualitative study explored graduate students' pathways of growth toward transdisciplinary thinking during one semester. The students were enrolled in a professional development course as part of a training grant. We examined artifacts from the course through multiple rounds of thematic coding. We used a grounded theory approach to gain insights into how graduate students navigate their journey toward transdisciplinary thinking. We propose a model of this journey. It traces students' trajectories from prior experiences, through reflexive thinking about several crucial skills and dispositions, towards a transdisciplinary "prism" that transforms their thinking. We conclude that becoming a transdisciplinary researcher is not a matter of acquiring competencies, but a life-long process. Fostering this process may require fundamental reimagining of graduate education.