The purpose of this study was to develop a valid and theoretically sound 21st century skills-based mathematics learning model. Plomp's research and development theory and Nieveen's criteria were used to ensure the quality and reliability of the model. The model development process includes Initial Investigation, Design, and Realization/Construction. Data were collected from six schools through various instruments such as observation sheets, documents, 21st century skills tests, and interview guides. Model validation involved the participation of five experts. A blended data analysis approach was used to ensure a comprehensive understanding of the findings. The result of this study is a 21st century skills-based mathematics learning model named Model 21-BML. The model consists of five interconnected core components: Learning Syntax, Social System, Reaction Principles, Support System, and Instructional and Accompanying Impacts. The uniqueness of this model lies in the seven integrated learning trajectories: Object translation, decomposition, cognitive construction, cognitive development, communication, reflection, and extension. After going through a rigorous validation process, the five experts confirmed the high validity of the 21-BML Model. This model not only enhances innovative mathematics learning theory, but also provides practical alternatives for mathematics teachers to encourage the development of 21st century skills in students. The model is expected to make a significant contribution in bridging the gap between traditional mathematics education and the competency demands of the modern era, as well.