The purpose of this study is to compare and contrast student, teacher, and school factors that are associated with student mathematics achievement in South Korea and the United States. Using the data from the Trends in International Mathematics and Science Study (TIMSS) 2011, this study examines factors that are linked to teachers who deliver quality instruction with high self-efficacy in both countries. We also investigate the association between teachers with high-quality instruction and high self-efficacy and 8th graders’ mathematics achievement. It was found that teachers’ perceived academic emphasis was commonly associated with teachers who claimed to provide high-quality mathematics instruction with high self-efficacy. However, the two countries’ results differed in the association between teachers’ opportunities to learn in professional development programs and high-quality instruction with high self-efficacy. While ‘teachers’ opportunities to learn in professional development programs’ was a significant factor that leads to quality instruction and high self-efficacy in the United States, it was not significant in South Korea. In addition, while students in the United States tended to have higher math achievement scores when their teachers delivered high-quality instruction with high self-efficacy, there was no such association in South Korea.