This article presents a design for supporting theory-practice reflection in teacher practicums. This design is based on three design principles that promote a transformative stance towards the creation of novel pedagogical approaches: mutual transformation of theory and practice, co-design among supervising teachers, university lecturers and student teachers, and participating in the long-term development of pedagogical practices. To evaluate the design, we analyse and compare student teachers' portfolios across two implementations of the teacher practicum: a thematic practicum based on the proposed design and the conventional class teacher practicum of the University of Helsinki. The results show that the student teachers who participated in the thematic practicum used theory more frequently in their reflections. Moreover, their theory-practice connections were more robust than those made by student teachers who participated in the conventional practicum. On the basis of our results, we make suggestions regarding how to promote theory-practice reflection in teacher education.