Background: Professional self-concept is one of the important outcomes of nursing professionalism. There is a lack of adequately planned curriculum may limit nursing students’ practical knowledge, skills and professional self-concept in providing comprehensive geriatric-adult care and promoting nursing professionalism. Professional portfolio learning strategy has allowed nursing students to continue professional development and enhance nursing professionalism in professional clinical practice. However, there is little empirical evidence in nursing education to support the use of professional portfolios in blended learning modality among internship nursing students. Therefore, this study aims to examine the effect of the blended professional portfolio learning on professional self-concept among undergraduate nursing students during Geriatric-Adult internship. Methods: A quasi-experimental study two-group pre-test post-test design. A total of 153 eligible senior undergraduate students completed the study (76 in the intervention group and 77 in the control group). They were recruited from two Bachelor of Sciences in Nursing (BSN) cohorts from nursing schools at Mashhad University of Medical Sciences (MUMS), in Iran, in January 2020. Randomization was undertaken at the level of school via a simple lottery method. The intervention group received the professional portfolio learning program as a holistic blended learning modality, though the control group received conventional learning during professional clinical practice. A demographic questionnaire and the Nurse Professional Self-concept questionnaire were used for data collection. Results: The findings imply the effectiveness of the blended PPL program. Results of Generalized Estimating Equation (GEE) analysis was indicated significantly improved professional self-concept development and its dimensions (self-esteem, caring, staff relation, communication, knowledge, leadership) with high effect size. The results of the between-group comparison for professional self-concept and its dimensions at different time points (pre, post and follow up test) showed a significant difference between groups at post-test and follow up test (p < 0.05),while at pre-test there was no important dissimilarity between two groups (p > 0.05).The results of within-group comparison for both control and intervention showed that there were significant differences in professional self-concept and for all its dimensions across the time from pre-test to post-test and follow-up (p < 0.05), and also from post-test to follow-up it was significant (p < 0.05) for both groups. Conclusion: This professional portfolio learning program demonstrates as an innovative and holistic blended teaching-learning approach to improve professional self-concept during professional clinical practice among undergraduate nursing students. It appears that the use of a blended designed of professional portfolio can promote a link between theory and the advancement of geriatric adult nursing internship practice. The data obtained from the present study can be useful for nursing education to evaluate and redesign a curriculum for development of nursing professionalism as a quality improvement process and groundwork to develop new models of teaching-learning and assessment.