Clinical reasoning has been studied in residents or nurses, using interviews or patient-provider encounters. Despite a growing interest in interprofessional collaboration, the notion of collaborative reasoning has not been well studied in clinical settings. Our study aims at exploring resident-nurse collaborative reasoning in a simulation setting. We enrolled 14 resident-nurse teams from a general internal medicine division in a mixed methods study. Teams each managed one of four acute case scenarios, followed by a stimulated-recall session. A qualitative, inductive analysis of the transcripts identified five dimensions of collaborative reasoning: diagnostic reasoning, patient management, patient monitoring, communication with the patient, and team communication. Three investigators (two senior physicians, one nurse) assessed individual and team performances using a five-point Likert scale, and further extracted elements supporting the collaborative reasoning process. Global assessment of the resident-nurse team was not simply an average of individual performances. Qualitative results underlined the need to improve situational awareness, particularly for task overload. Team communication helped team members stay abreast of each other's thoughts and improve their efficiency. Residents and nurses differed in their reasoning processes, and awareness of this difference may contribute to improving interprofessional collaboration. Understanding collaborative reasoning can provide an additional dimension to interprofessional education.