This Design-Based Research (DBR) study investigated the adaptation of Realistic Mathematics Education (RME) for teaching Grade 9 mathematics in an Ethiopian secondary school context. The study addressed the adaptation gap between Ethiopia‘s Competency-Based Curriculum vision and classroom reality by developing, implementing, and refining RME-based lesson sequences through two iterative cycles. Cycle 1 focused on trigonometry using a flagpole context, while Cycle 2 focused on polygon exterior angles using a culturally specific metalworker context from Bahir Dar. Data were collected through multiple methods including, reflective journaling, student-generated artifacts, video and audio recordings of classroom discourse, photographs, and critical friend debriefing. Analysis was guided by the three core RME principles: Reality Principle, Guided Reinvention, and Progressive Formalization. Findings revealed that in Cycle 1, students independently reached the general level of progressive formalization, discovering the constant ratio in height and shadow measurements, but formalization remained teacher-directed with the introduction of ―tan A.‖ In Cycle 2, with improved design features including a culturally sustained context, structured data tables, and opportunities for student naming before formal terminology, all six groups completed the entire progressive formalization trajectory independently, articulating the formula 360° ÷ n in their own words, with one group using variable notation: ―Let ‗n‘ be number of sides.‖ Ten evidence-based design principles emerged for effective RME adaptation in Ethiopian secondary mathematics, including culturally sustained realistic context, eliciting student ideas without judgment, structured investigation with student ownership, pattern recognition before formalization, student naming before formal terminology, formula emergence from data patterns, authentic handling of measurement variation, cross-group comparison and discussion, application back to original context, and teacher as designer and summarizer. The study contributes empirical evidence of RME‘s effectiveness beyond its original Dutch context, elaborates the process of progressive formalization, and provides practical guidance for mathematics teachers, teacher educators, curriculum developers, and policymakers seeking to implement the Competency-Based Curriculum‘s vision of student-centered, contextually relevant mathematics instruction.