This study examines the direct and indirect relationships between key motivational constructs and science achievement using TIMSS 2019 data sets for Japan, Norway, and the United States. The sample of the study includes 4,446 and 8,698 eighth graders in Japan and the United States, with 4,575 ninth graders in Norway. The structural equation model was built on the self-determination theory (SDT) and the self-enhancement model of self-concept. The study's findings show that self-concept is consistently the strongest predictor of science achievement among motivational constructs for all three countries. In addition, the significant relationship between self-concept and autonomous motivation supports the SDT's claims about basic psychological needs. Although this relationship is consistent, as SDT predicts, across three countries, the magnitude of the relationship seems to be culture-dependent. The findings of this study are discussed using the SDT and self-concept theory from a cultural standpoint.