Background: Assertiveness in nursing is crucial to improving communication, reducing interpersonal tensions, and improving healthcare outcomes. Objective: The objective of this systematic review was to identify and synthesize the literature on assertiveness in nursing, focusing on nurses’ perspectives regarding its role, influencing factors, challenges, and impact within complex healthcare settings. Methods: This review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. An integrated mixed methods approach was used to capture the multifaceted nature of assertiveness in nursing. The evaluation covered both qualitative and quantitative investigations, concentrating on quantitative publications between 2018 and 2024 and qualitative publications without a time limit. Diverse methodologies were incorporated using the SPIDER framework. A comprehensive electronic search was conducted across six databases: PubMed, Medline, CINAHL, PsychInfo, Wiley Online Library, and Web of Science. Ultimately, 15 research papers were included in the review out of 9490 publications that were initially identified. Included studies were critically appraised using the Critical Appraisal Skills Program (CASP) and the Joanna Briggs Institute Critical Appraisal checklists. Results: A total of 9490 studies were identified, of which 15 (eight quantitative and seven qualitative) were included in this review. The review and subsequent analysis revealed five major themes: (1) facilitators of assertiveness; (2) barriers to assertiveness; (3) assertiveness training; (4) interaction with other staff; and (5) patient relationships. Conclusions: Assertiveness is essential for proficient nursing practice, especially in complex healthcare environments. It facilitates improved communication, alleviates stress, and augments patient care. Future research should explore the long-term benefits of assertiveness training and its influence across various cultural settings.