Due to international calls for the use of the integrated Science, Technology, Engineering, and Mathematics (STEM) approach to train learners pursuing STEM-related careers and citizens who are competitive problem solvers, teachers' training for the integrated STEM approach is imperative. Given the importance of integrated STEM teacher training, this study aimed to examine how pre-service chemistry teachers' integrated STEM conceptions have been changed through the integrated STEM course. The STEM reflection protocol was administered to 13 pre-service chemistry teachers both at the beginning and at the end of a 13-week-long integrated STEM course. The training included experiencing STEM activities, studying with mentors, and reflecting on own development. The deductive, inductive, and constant comparative data analysis revealed that most of the pre-service chemistry teachers enriched their STEM conceptions in terms of the engineering design process as a context and integration of the disciplines. However, very few participants' STEM conceptions remained the same. Integrated STEM teacher education should include an explicit emphasis on disciplines' integration and engineering design process to enhance pre-service teachers' integrated STEM conceptions.