This article reports on the development and validation of the Informal Second Language Engagement questionnaire (ISLE) for capturing various aspects of learner engagement with informal second language practices. Whereas other questionnaires have primarily focused on learner behavior (the frequency, quantity, and diversity of informal activities in which learners engage), the ISLE additionally targets affective, cognitive, and linguistic aspects of engagement. Consequently, this questionnaire has the potential to help second language acquisition researchers better understand individual differences and variability in informal language engagement, how it changes over time, and the ways in which engagement affects language learning. The ISLE was developed and evaluated in three stages with a total of 382 German secondary school learners of English as a foreign language. The initial instrument development was informed by qualitative findings from focus group interviews. Subsequently, the questionnaire was piloted, and Exploratory Factor Analysis was applied to the data to uncover the underlying factor structures and reduce the number of items per construct. Finally, the revised instrument was evaluated further using Confirmatory Factor Analysis.