Science, technology, engineering, and mathematics (STEM) education is an international focus in teaching and learning, and factors influencing students' science achievement vary throughout the world. Guided by the Social Role and Expectancy-Value Theories, we investigated the role of fourth and eighth graders' gender and attitudes toward science in science achievement in the Trends in International Mathematics and Science Study (TIMSS) 2015. In this study, we focused on South Korea, Turkey, and the United States, where students' science achievement scores varied substantially in TIMSS 2015. Findings indicated that having a positive attitude toward science positively predicted students' science achievement scores in fourth and eighth grades. Students' science scores varied slightly but significantly between male and female students in South Korea and Turkey, controlling for their attitudes toward science. Longitudinal studies of science achievement may be included in further research.