This study aims to examine the effects of peer coaching on the development of Preservice Science Teachers' (PSTs) Technological Pedagogical Content Knowledge (TPCK) and the components of TPCK. A total of 37 pre-service science teachers (27 females and 10 males) participated in the study. Of the different types of coaching that are available, "mutual peer coaching" was used in this study. In the first two weeks of the 10-week research process, the PSTs received peer coaching training. In the remaining 8 weeks, the PSTs were asked to implement peer coaching in their classroom teaching. In addition, one-hour theoretical classes were taught each week in the "Teaching Practice" course, during which the PSTs received feedback on their teaching practices and peer coaching forms. Therefore, a total of 296 forms were examined in this study. The data were analyzed through content analysis. They were examined in three categories: 1-Scientific explanation, 2-Partially scientific explanation, and 3-Non-scientific explanation. The findings showed an improvement in the PSTs' TPCK and its components. It was found that during the preliminary interviews in the early weeks, the PSTs had partially scientific explanations in the three content areas (content knowledge, pedagogical knowledge and pedagogical content knowledge), and non-scientific explanations only in technological knowledge and technological pedagogical content knowledge. In the later weeks, they moved towards scientific explanations in pedagogical knowledge, technological knowledge and technological pedagogical content knowledge. In the observation of their teaching, the PSTs had partially scientific explanations in all five content areas in the first weeks, and shifted towards scientific explanations in later weeks. In the final interviews, the PSTs were found to move from nonscientific explanations towards partially scientific explanations in four content areas. On the other hand, only in pedagogical knowledge, students who had partially scientific explanations in the first weeks remained at the same level in the later weeks.