The purpose of this study was to develop a hypothetical learning trajectory and related instructional sequence for proportional reasoning based on the theory of Realistic Mathematics Education. In line with this purpose, the hypothetical learning trajectory and related instructional sequence were developed, tested, revised, and finalized in three macrocycles conducted in seventh grade classrooms through a design-based research perspective. The classroom data that emerged during the instruction in the third macrocycle were analyzed by Toulmin's model of argumentation. Findings revealed that teaching ratio and proportion by progressing from informal to formal tools in realistic contexts has significant potential for students to learn this subject in meaningful, comprehensive, and coherent ways. Within the context of this study, in addition to analyzing a classroom community's learning, the means used to support that learning are being portrayed. In this regard, it is foreseen that the results of this study related to design and implementation would be useful for improving the quality of the teaching and learning of ratio and proportion.