Background: The Accreditation Council for Graduate Medical Education (ACGME) Milestone projects required each specialty to identify essential skills and develop means of assessment with supporting validity evidence for trainees. Several specialties rate trainees on a milestone subcompetency related to working in interprofessional teams. A tool to assess trainee competence in any role on an interprofessional team in a variety of scenarios would be valuable and suitable for simulation-based assessment. Objective: We developed a tool for simulation settings that assesses interprofessional teamwork in trainees. Methods: In 2015, existing tools that assess teamwork or interprofessionalism using direct observation were systematically reviewed for appropriateness, generalizability, adaptability, ease of use, and resources required. Items from these tools were included in a Delphi method with multidisciplinary pediatrics experts using an iterative process from June 2016 to January 2017 to develop an assessment tool. Results: Thirty-one unique tools were identified. A 2-stage review narrowed this list to 5 tools, and 81 items were extracted. Twenty-two pediatrics experts participated in 4 rounds of Delphi surveys, with response rates ranging from 82% to 100%. Sixteen items reached consensus for inclusion in the final tool. A global 4-point rating scale from novice to proficient was developed. Conclusions: A novel tool to assess interprofessional teamwork for individual trainees in a simulated setting was developed using a systematic review and Delphi methodology. This is the first step to establish the validity evidence necessary to use this tool for competency-based assessment.