It was aimed in this study to design a lesson, carried out with pre-service science teachers, for the use of the portfolio based on the feedback process in the teaching of classroom assessment methods and to reveal the applicability of this lesson. In this study, carried out using qualitative research methods, an action research design was used. A total of 12 pre-service science teachers were the participants of the study. The study was conducted in the Assessment of Classroom Learning elective lesson. Data in this study were obtained from reflective writings of the preservice teachers and interviews conducted with them. Content analysis and descriptive analysis methods were used in the analysis of the study data. As a result of the study, the lesson designed to teach in-class assessment methods was found to be effective and beneficial by the pre-service teachers, and in particular, the feedback given to them was noted to be effective in assisting them to see their own mistakes and correct these mistakes and they reported seeing the improvement in themselves throughout the portfolio process. The study, which was carried out by using a portfolio supported by feedback, presented an exemplary lesson model that can be used in the teaching of classroom assessment methods. [Note: The publication year (2022) shown on the PDF is incorrect. The correct publication year for this article is 2023.]