Pain has been conceptualized globally as the "fifth vital sign" and is widely acknowledged as a significant clinical indicator. The purpose of this study was to evaluate how nurses in the Nigerian city of Ilorin perceived pain as the fifth vital sign. A mixed-methods convergent parallel design was used. Semi-structured interviews with 17 purposefully chosen nurses yielded qualitative insights, while structured questionnaires were used to gather quantitative data from 343 registered nurses in chosen healthcare facilities. Together with thematic content analysis, descriptive and inferential statistics were used to analyze the data. Nurses agreed that routine assessment improves patient outcomes and strongly agreed that pain is an important vital sign (mean scores 3.58–3.92). Though pain was conceptually acknowledged by nurses as the fifth vital sign, its clinical application was uneven, indicating a theory–practice gap. Assessments of pain were frequently situational, given priority in acute care and surgical settings but disregarded in routine examinations. Implementation was further limited by institutional obstacles like a lack of standardized procedures, a high workload, and a staffing shortage. Although nurses in Ilorin City exhibit a high level of awareness regarding the significance of pain as the fifth vital sign, they consistently encounter institutional and systemic obstacles in their work. Standardized institutional protocols, organized professional development programs, and supportive policies that incorporate pain into routine vital sign assessment are necessary to strengthen pain assessment in nursing practice. Improving patient comfort, results, and the provision of holistic care requires bridging the perception-practice divide. Keywords: Nursing practice, Perception, Pain assessment, Fifth vital sign,