The role of the mathematics textbook in times of resource diversity Mathematics textbooks used to be the key resource for students’ self-regulated learning of mathematics. Primarily due to the digitalization of society, students have potentially greater access to a wider range of resources such as internet search engines, learning platforms, educational videos, and Generative AI. This study investigates the role of the mathematics textbook in comparison to other resources within students’ self-regulated learning practices. Data were collected via a survey of 1101 German secondary students, representing three school types (Gymnasium, Gesamtschule, Realschule) and three grade levels (6, 9, and upper secondary). The questionnaire assessed the frequency of resource use in and outside class, reasons and purposes of use, and resource-based strategies when facing learning challenges outside class. Results show that the printed mathematics textbook is the most frequently used resource both in class and outside class. The textbook remains the most relevant resource for key purposes, such as an aid for doing homework and preparing for tests and exams. However, its dominance diminishes with age: in upper secondary school, students increasingly rely on self-created notes, and online resources. Correlation analyses reveal moderate to strong links between in-class and out-of-class use, suggesting an association between resource use and classroom culture. The findings underscore the textbook’s enduring centrality as a foundational, trusted resource within a dynamic and increasingly diverse learning environment. This study calls for pedagogical approaches that integrate textbooks more intentionally within broader resource systems, supporting students’ agency and strategic resource selection in an era of digital abundance. Springer