This study aims to improve science teachers' scientific reasoning skills (SRS) in using and teaching these skills through a professional development program, Scientific Reasoning Skills Teacher Training Program (SRSTP). Forty-five middle school science teachers participated in the study, which was on convergent parallel design. "Scientific Reasoning Skills Assessment Form (SRSAF)" and "Scientific Reasoning Skills Test for In-service and Pre-service Science Teachers (SRSTIPST)" were used to determine the improvement in teachers' use of scientific reasoning skills. Besides, "Self-efficacy Perceptions towards Teaching Scientific Reasoning Skills Assessment Form (SEPSRSAF)" and "Self-efficacy Perceptions towards Teaching Scientific Reasoning Skills Scale (SEPSRS)" were used to determine teachers' self-efficacy perceptions towards teaching them. Findings from SRSAF and SRSTIPST pointed out that teachers' scores in using specific scientific reasoning skills and their ways of making claims, presenting evidence, and reasoning differed significantly after SRSTP. Findings from SEPSRS showed that teachers got significantly higher scores in creating SRS based learning environment, academic proficiency, using SRS in the classroom, assessment of SRS, and instructional ways for teaching SRS after the professional development program. SEPSRSAF supported these findings by revealing that SRSTP allowed teachers to change their efficacy sources from indirect experience to active experiences and improve personal characteristics such as showing empathy. It was also found that teachers' perceptions of teaching SRS shifted towards teacher-related factors after SRSTP. These findings were discussed, and the contribution of the results was explained.