The aim of this study was to quantify the professionalism of nurses in Japan. The Japanese version of the Behavioural Inventory for Professionalism in Nursing was conducted as a national survey. Computer-generated random selection of nurses in Japan obtained responses from 1501 nurses. A descriptive design examined the levels of and differences in nursing professionalism. Comparisons of the total level of professionalism in educational preparation, current position, years of experience, and current practice setting were analysed by one-way analysis of variance and post hoc Tukey–Kramer multiple comparison test. The results revealed that Japanese nurses had low levels of professionalism, and professionalism was related significantly to higher educational preparation, years of experience as a nurse, and current position as a nursing administrator or faculty. The results can be used as a benchmark for continued assessments of the level of nursing professionalism and for further development of nursing professionalism.