Research shows that motivation is a critical precursor to students' science achievement. However, research on the topic yields ambiguous results in different cultural contexts. While the positive association between motivation and science achievement has been evidenced universally, the relatively predictive effect and strength of different motivational factors in different cultures need additional examination. To tease the potential discrepancy apart and explore different student motivational factors including self-concept, intrinsic value, utility value, and sense of relatedness, this study examines grade 8 students from the U.S. (N = 14,290) and Singapore (N = 6113) by using the International Trends in Mathematics and Science Study (TIMSS) data. The results suggest that the U.S. student self-concept in science learning is the strongest and most significant predictor of their science achievement. In Singapore, students perceived utility value in science learning has the strongest and most significant association with their science achievement. Explanations of the differences as well as the implications for future research are discussed.