This study aims to examine the impact of engaging in reflective experiences on one’s own teaching through teaching portfolios during a teaching practicum on the agency of preservice science teachers. To achieve this, we instructed 20 pre-service science teachers who participated in the teaching practicum in May 2024 to create teaching portfolios. These portfolios contained classroom discourse analyses and self-evaluations of lessons from the early, middle, and final stages of the practicum. We assessed agency at both the onset and conclusion of the practicum, and subsequently carried out in-depth interviews to help interpret the findings. The findings of the study are as follows. First, even shortterm reflective activities based on teaching portfolios led to significant changes in teacher agency. Second, the pre-service teachers who critically reflected on their lessons developed a stronger awareness of themselves as agents of their teaching. These teachers were able to enhance their ability to make creative decisions when selecting and reorganizing instructional materials.