Diagnosis improvement requires physician-nurse collaboration. This study explored nurses’ concerns regarding physicians’ diagnoses and how they were communicated to physicians. This cross-sectional study, employing a web-based questionnaire, included nurses registered on Japan’s largest online media site from June 26, 2023, to July 31, 2023. The survey inquired whether participants felt concerned about a physician’s diagnosis within a month, if they communicated their concerns once they arose, and, if not, their reasons. The reasons for not being investigated were also examined. The nurses’ frequency of feeling concerned about a physician’s diagnosis and the barriers to communicating these concerns to the physician were evaluated. Overall, 430 nurses answered the survey (female, 349 [81.2%]; median age, 45 [35–51] years; median years of experience, 19 [12–25]). Of the nurses, 61.2% experienced concerns about a physician’s diagnosis within the past month; 52.5% felt concerned but did not communicate this to the physician. The most common reasons for not communicating included concern about the physician’s pride, being ignored when communicating, and the nurse not believing that a diagnosis should be made. Our results highlight the need to foster psychologically safe workplaces for nurses and create educational programs encouraging nurse involvement in diagnosis.