This study aimed to examine the mediating effects of motivational beliefs in the relations between parental involvement and science achievement. Three types of motivational beliefs, namely, self-concept, intrinsic value, and utility value were addressed based on the Expectancy-Value Theory model. A representative national sample from Taiwan of 5042 eighth-grade students with an average age of 14.2 years was examined. The results indicated that self-concept and utility value can mediate the effects of parental involvement on science achievement, whereas intrinsic value does not have such a mediating effect. These findings provide empirical evidence revealing the prominent role of parental involvement in students’ science achievement through a motivational mechanism.