To be successful in learning, students need to be motivated to engage and learn. The domain-specificity motivation theory articulates that student motivation is often determined by the content being taught to them. The purpose of this study was to extend the theory by determining domain-specificity of situational interest and expectancy-value motivation in terms of engagement and achievement outcomes in physical education. A random student sample (N = 346) from eight Chinese middle schools provided data of situational interest, expectancy-value, engagement, and knowledge and skills acquired. Results from correlation, regression, and structural equation model analyses revealed causal inferences demonstrating differentiated effects of motivation components on the outcome measures: task values were specific to knowledge outcome, expectancy beliefs to skills, and situational interest to engagement. The findings imply that physical educators need to adopt motivation strategies compatible to specific learning outcomes to maximize student motivation for engagement and achievement.