Purpose: This research aims to examine the pre-service teachers' learning when using STEM tasks with middle school students in a lesson study. Design/methodology/approach: This research adopted a qualitative methodology with an interpretative orientation. The participants are three pre-service teachers involved in a lesson study that took place in a curricular unit of a master's program in physics and chemistry teaching. Data were collected through observation and written reflections. Findings: This lesson study, conducted as part of initial teacher training, allowed pre-service teachers to deepen their understanding regarding students' difficulties when engaged in a STEM task involving prototype construction. It also enabled them to learn about the engineering design process and the integration of the four STEM areas. Reflecting with peers and educators on the research lesson, taught by the cooperating teacher, and on the results allowed them to develop knowledge about the use of STEM tasks. Originality/value: This research highlighted the importance of introducing STEM education in the initial teacher training. Specifically, this research contributed to the scarce studies on lesson studies with pre-service teachers that focus specifically on the use of STEM tasks.