Purpose: With the belief that situational interest (SI) can be characterized as being "triggered" and "maintained," the authors conducted this study to identify the underlying structure of SI and to develop a new measurement in physical education. Method: There were 558 students from two urban high schools in Shanghai, China, who served as participants. The authors developed the Situational Interest Inventory-Physical Education through three systematic stages. Both exploratory factor analysis and confirmatory factor analysis were conducted. Results: An exploratory factor analysis provided preliminary support, with a model comprising three components: triggered SI, maintained SI feeling, and maintained SI value. The three-component model was further corroborated through a confirmatory factor analysis. Its predictive validity was supported with significant correlations to in-class engagement. Conclusion: The findings lend initial evidence to the theoretical mechanism of interest development. Clarifying how SI is related to the mode of teachers' instruction and learning content may help design effective motivational strategies and nurture long-term individual interest.