In Japan, the clinical nurse instructor is a staff nurse who teaches in clinical practicums. However, there is no consensus on the essential roles that clinical nurse instructors are expected to perform. We conducted a three-round Delphi survey to clarify the essential roles of the clinical nurse instructor in clinical practicums in undergraduate nursing education. The participants were an expert panel of 48 professionals in nursing education and clinical practicums, who rated the importance of 58 role items that were established through a literature review and pilot survey. Thirty one of these items were identified as essential roles, based on agreement of 80% or more of respondents. Further investigation revealed nine of the 31 items to be core roles, defined as the minimum essential roles that must be performed by clinical nurse instructors, however busy they become. The nine core roles are related to proper preparation for the clinical practicum, patient safety, and coordination with the nursing school faculty. It is important for the nursing school faculty to support and work in cooperation with clinical nurse instructors to help them fulfill these roles.