Sustainability is a foundational principle in Chilean education, reflected in curricular objectives related to environmental care, economic development, and social well-being. This study analyzes the integration of sustainability concepts and dimensions into the curricular bases of the third year (11th grade) and fourth year (12th grade) in Chilean secondary education. Using a sequential explanatory mixed-methods design and content analysis, the quantitative phase identified six key sustainability-related terms and their presence across curricular components and subject areas. The qualitative phase examined the inclusion of the environmental, social, and economic dimensions within those areas. The results show that sustainability concepts appear in seven subject areas, with greater emphasis on learning objectives and educational purposes. However, the environmental dimension dominates, while the social and economic aspects are underrepresented. These findings reveal conceptual ambiguities and uneven integration, highlighting challenges for implementing a multidimensional sustainability approach in Chilean classrooms.