Background: The integration of science and engineering practices in K-12 science education provides a platform for students to engage in productive classroom discourse supporting deep student understanding and reasoning in a real-world context. However, the engineering design discourse in K-12 science classrooms has not yet been fully examined. Purpose: This article aims to explore the decision-making processes and verbal interactions of eighth grade students as they engage in a 12-day engineering design-based science unit. The following research questions guided the study: (a) How do student discourse patterns and interactions affect design decisions? (b) How do the instructor's discursive interactions in design groups influence student design? Method: In this exploratory case study, discursive patterns and interaction patterns between four student groups and their instructor were captured through audio and video recordings. Results: The design discourse structure and patterns differed among groups, affecting their design decisions. There was also a difference in both the number of new ideas generated and, most notably, the level of student engagement in those groups. In addition, the complexity of design decisions varied, with most groups focusing on financial feasibility rather than scientific reasoning. Finally, it was observed that the instructor discourse influenced student engagement in design discourse and the use of scientific reasoning and justifications in small group discussions. Conclusions: This discourse study offers insight into students' scientific reasoning in an effort to help educators more effectively structure instruction discourse and practices in engineering design in K-12 science education.