<p>Research on the interaction between curricula and textbooks—particularly the textbook development process—is critical for understanding their reciprocal influence. While mathematics textbooks are typically developed in response to curriculum revisions, textbook research can also inform and shape subsequent changes to the curriculum. This study investigates how textbook research in Korea, with a focus on the equal sign and students’ understanding of it, has contributed to the development of a new curriculum standard. It examines the background and rationale for introducing this new standard on the equal sign and equivalence, an unusual addition to an already packed curriculum. Drawing on analyses of official policy documents and nine government-approved textbooks, this study illustrates how textbook research was incorporated into the revised curriculum and how the new standard was subsequently reflected in textbook design. As such, the study presents a unique case of the dynamic interplay between textbook research, curriculum development, and textbook implementation, highlighting the potential of textbook research to drive curriculum change.</p>