Purpose: In the 21st century, the importance of the fields of science, mathematics, technology, and engineering, and the need of the skilled individuals have paved a way for a great responsibility on teachers providing instruction in these four fields. Therefore, in this context, it is essential that teachers need to receive education for the cultivation of STEM skills in schools. The present study, it is aimed investigate the development of the STEM-focused lesson planning skills of pre-service science teachers who participated in STEM-focused laboratory activities. Research Methods: This research was designed as a single case study. The participants in this research study were seven pre-service science teachers. Data were collected through the lesson plans developed by the pre-service science teachers and analyzed using descriptive analysis. Findings: The results showed that STEM-focused laboratory activities contributed to the development of pre-service science teachers' skills about planning a STEM-focused lesson. Implications for Research and Practice: The findings suggest that the Science Teaching Laboratory Practices course might be structured with STEM-focused activities to improve the STEM-focused lesson planning skills of pre-service teachers. Providing long-term education to pre-service teachers in person can be recommended for improving their STEM-focused lesson planning skills. In this research, lesson plans of pre-service teachers were examined in terms of content, approach, assessment and evaluation. Development of more detailed analyses is suggested for examining lesson plans.