Background: Science teacher education programs are considered to play a critical role in preparing future science teachers who are more innovative and capable of effectively implementing integrated STEM education. Purpose: This research investigates the effectiveness of a professional development program enriched by the skills-based science, technology, engineering, and mathematics model (SB-STEM-M) on preservice science teachers' self-efficacy. Sample: The participants of this study are twenty preservice teachers in a state university science education program. Design and Methods: This research was carried out over 14 weeks using mixed-methods research, with the implementation process consisting of three stages, with the last stage involving the preservice teachers' preparation of SB-STEM activity sets alongside evaluations, and feedback. The quantitative data in the research were obtained using the Self-Efficacy and Concerns about STEM Education Instrument, which was used as a pre-and post-test. The qualitative data were obtained through the activity sets and videos the preservice teachers prepared based on SB-STEM-M as well as through interview forms and focus group interviews. Results: The results from the quantitative and qualitative data analyses show the preservice teachers' STEM self-efficacy to have improved using the professional development program enriched with SB-STEM-M, while no change had occurred regarding their STEM concerns like information and evaluation, management, consequence and refocusing. Conclusion: The three-stage intervention process applied to the preservice teachers enabled them to both develop original STEM activity sets in accordance with SB-STEM-M as well as to increase their STEM self-efficacy. The activity sets and videos the preservice teachers prepared reveal evidence that they had achieved the capacity to have their classrooms apply the activity sets in their future professional life. The data obtained through the interview forms and focus group interviews reveal them to have learned to integrate scientific process and reasoning skills into STEM disciplines.