It is important to understand how students reason in K-12 integrated STEM settings to better prepare teachers to engage their students in integrated STEM tasks. To understand the reasoning that occurs in these settings, we used the lens of collective argumentation, specifically attending to the types of warrants elementary students and their teachers provided and accepted in integrated STEM contexts and how teachers supported students in providing these warrants. We watched 103 hr of classroom instruction from 10 elementary school teachers and analyzed warrants that occurred in arguments in mathematics, coding, and integrated contexts to develop a typology of warrants contributed in mathematics and coding arguments. We found that these students made their warrants explicit the majority of the time, regardless of the teacher's presence or absence. When teachers were present, they supported argumentation in various ways; however, they offered less support in integrated contexts. Additionally, we found students relied more on visual observations in coding contexts than in mathematics or integrated contexts, where they often provided warrants based on procedures required to accomplish a task. These findings have implications for improving integrated STEM instruction through engaging students in argumentation.