The transformation of 21st-century education demands a paradigm shift in learning from teacher-centered to student-centered learning. Critical thinking, creativity, communication, and collaboration are among the competencies that need to be developed as part of these 21st-century competencies. However, mathematics learning in elementary schools is often oriented towards memorizing procedures, making it difficult for students to deeply understand concepts, particularly in algebra. This study aims to describe the implementation of Deep Learning-based algebra learning in developing 21st-century competencies in elementary school students. The study used a qualitative descriptive approach with 28 fifth-grade elementary school students as subjects. Data were obtained through observation, documentation, student worksheets, formative assessments, and learning reflections. The results showed that the application of Deep Learning through the stages of stimulation, concept exploration, knowledge construction, collaboration, contextual application, reflection, and learning transfer was able to improve understanding of algebra concepts and develop students' critical thinking, creativity, communication, and collaboration competencies. Learning that links variable concepts to real-life situations helps students build more meaningful conceptual understanding. Furthermore, group discussions and reflection activities encourage students to be more active in expressing opinions, collaborating, and evaluating their thinking processes. Research findings indicate that Deep Learning-based algebra learning can be a strategy for implementing the Independent Curriculum (Kurikulum Merdeka) to prepare students for 21st-century challenges.