This study investigated gender disparities in students’ science motivational beliefs, conceptualized as self-concept and intrinsic value, from Grades 4 to 8 in 22 countries. The gender-specific relation between science motivational beliefs and achievement was also scrutinized. Data from the 2011 and 2015 national cohorts of the Trends in International Mathematics and Science Study (TIMSS) were explored. The t test and effect size results indicated gender gaps in self-concept and intrinsic value at the two grade levels. Boys’ motivational beliefs were stronger than girls’ in Grade 8 in the majority of countries, but the pattern in Grade 4 varied across countries. The relatively high science motivational beliefs among girls in Middle Eastern countries are extensively discussed from sociocultural perspectives. A downward trend of science motivational beliefs over grade levels occurred in all countries, and the degree to which girls’ motivational beliefs decreased was greater than the decrease for boys in most of the countries. Moreover, the regression modeling results showed that boys’ self-concept and intrinsic value accounted more for their science achievement than did girls’ in both grades. Students’ self-concept was a stronger predictor than intrinsic value of achievement, regardless of gender. The findings provide empirical evidence from multiple countries of the development trends of science motivational beliefs, with gender as the focus.