Research and practice have recognized the importance of informal learning--a specific type of active learning--for higher education contexts. University students learn not only in formally organized courses, but also in a self-directed and intentional way from fellow students, through trial and error, and by reflection. However, there has been a lack of valid measures to operationalize students' informal learning. In this study, we thus develop the Informal Student Learning (ISL) scale for higher education, building on the Octagon Model of Informal Workplace Learning and the model's associated measure from the vocational learning context. Our scale contains eight components with three items each. Using three samples of university students (N = 545, N = 818, N = 310), we examined the model structure of ISL and the scale's validity in an international context. The results show that the conceptual structure of informal "workplace" learning and informal "student" learning is similar for "intent to learn" and "reflection," but different for "experience/action" and "feedback." Examining validity, we found evidence for concurrent validity in terms of study-related self-efficacy and academic performance, and for convergent and discriminant validity indicators of the eight ISL components. The scale has configural and metric measurement invariance for age, gender, and academic achievement, and additionally scalar invariance for age. Potential applications of the new measure in the context of active learning for research, for lecturers, and for students are discussed.