Purpose: This study aimed to identify the associations between job stress, social support, and nursing professionalism among nurses taking postgraduate courses, and to explore the factors influencing their nursing professionalism. Methods: The participants were 121 nurses who were enrolled in graduate school master's or doctoral programs in Korea. Data were collected from June 30 to August 31, 2021, using an internet-based self-report questionnaire. The data were analyzed by descriptive statistics, independent t-test, one-way ANOVA, Pearson's correlation coefficient, and multiple regression analysis using the IBM SPSS 23.0 program. Results: The mean scores of job stress, social support, and nursing professionalism among the participants were 2.47 out of 4, 4.06 out of 5, and 3.47 out of 5, respectively. There were statistically significant differences in nursing professionalism among the participants according to the degree program (F=-2.76, <i>p</i>=.007) and the length of nursing career (F=4.35, <i>p</i>=.015). Their nursing professionalism showed a positive correlation with social support (r=.45, <i>p</i><.001) and a negative correlation with job stress (r=-.43, <i>p</i><.001). As a result of the multiple regression analysis, factors influencing nursing professionalism of the participants were found to be job stress (β=-.28, <i>p</i>=.003), social support (β=.23, <i>p</i>=.015), and the length of nursing career (β=.19, <i>p</i>=.018). The explanatory power of the model was 28.0% (F=12.62, <i>p</i><.001). Conclusion: The present study suggests that organized efforts are needed to alleviate job stress and to increase social support for nurses in postgraduate courses in order to ultimately improve their nursing professionalism.