Science, technology, engineering, and math (STEM) education is of global focus, demanding qualified mathematicians, technologists, engineers, and scientists from diverse settings. Addressing the STEM academic achievement gaps among secondary students from marginalized backgrounds is a central concern and focus addressed in my research because of their lower participation rates in STEM, which limits not only their social versatility but limits critically important diverse perspectives in STEM fields. This includes girls, students of marginalized ethnic backgrounds, English language learners, and students from lower socioeconomic environments. My study explored whether the levels of a secondary teacher's self-efficacy play any role in their cultural competence and culturally responsive instructional practices and whether these play a role in the teacher's ability to serve as a conductor for positive STEM education for all students. Specifically, this mixed-methods convergent questionnaire variant research design aimed to explore whether secondary science teachers' level of STEM self-efficacy had any role on their level of cultural responsiveness and outcome expectancy beliefs regarding students from marginalized backgrounds. Using convenience sampling, I gained research setting approval to attend professional development district workshops to recruit volunteer teacher participants who worked in secondary science classes. The research findings revealed that secondary science teachers' STEM self-efficacy plays a role on their levels of cultural responsiveness and outcome expectancy beliefs toward their STEM students from marginalized backgrounds. The findings included teacher recommendations to enhance self-efficacy, cultural responsiveness, and expectancy beliefs. My study closes with a discussion of implications for practice and recommendations for future research. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]