Three studies were conducted to develop and validate scores on a new measure appropriate for assessing adolescents' situational interest (SI) across various academic settings. In Study 1 (n = 858), a self-report questionnaire was administered to undergraduates in introductory psychology. Confirmatory factor analyses (CFA) supported a three-factor model that differentiated between interest generated by (a) the presentation of course material that grabbed students' attention (triggered-SI), (b) the extent to which the material itself was enjoyable and engaging (maintained-SI-feeling), and (c) whether the material was viewed as important and valuable (maintained-SI-value). CFA analyses in Study 2 (n = 284) and Study 3 (n = 246) also supported the three-factor situational interest model for middle and high school students in mathematics. Moreover, situational interest was shown to be distinct from individual interest and was a statistically significant predictor of change in individual interest across the school year. (Contains 6 tables and 1 figure.)