This research study empirically tests: 1) the relationship(s) between stakeholder characteristics and project performance; 2) the mediating effect of effective stakeholder management strategies on the aforementioned relationship(s); and 3) the moderating effect of project complexity on the relationship(s) between effective stakeholder management strategies and project performance. To achieve the study’s objectives, a research model was proposed comprising four main components, namely stakeholder characteristics, effective stakeholder management, project performance and project complexity. These components were linked by 20 proposed hypotheses that were established according to assumed theoretical relationships. To validate the research model and test the proposed hypotheses, a quantitative research method was adopted. A series of multivariate statistical approaches were employed for the data, which were collected from 136 project managers and project management team members across diverse fields in more than 20 countries. Exploratory factor analysis was performed to identify factor structures that represented each underlying construct. Legitimate behaviour, supportive behaviour and opposing behaviour were identified as underlying factors of stakeholder power. Similarly, the agile response to change construct had two (internal and external agile response to change) underlying factors that represented the ability to respond to internal and external changes, respectively. Finally, quantitative and qualitative performance were revealed as underlying factors of project performance. Confirmatory factor analysis was then conducted to validate the results obtained from the exploratory factor analysis results. Structural equation modelling was then applied to test the hypotheses for the structural models. A number of key findings have emerged from the current study. Stakeholder characteristics have been proven to affect project performance. For instance, both legitimate behaviour and stakeholder interests negatively affect the ability to achieve quantitative project performance measures (i.e., schedule and budget), while opposing behaviour negatively affects both quantitative and qualitative project performance measures. Opposing behaviour negatively affects qualitative project performance, and the negative effect of opposing behaviour on qualitative project performance is mediated by either effective stakeholder management, internal agile response to change or strategic project flexibility. In particular, strategic project flexibility and effective stakeholder management fully mediate the negative effect of opposing behaviour and qualitative project performance. These findings indicate that the negative effect of opposing behaviour on qualitative project performance can be eliminated by implementing strategic project flexibility only or all three strategies together. Among the effective stakeholder management constructs, internal agile response to change was found to be the most influential factor for project performance; it positively affects both quantitative and qualitative project performance. Unfortunately, project complexity negatively moderates the positive effect of internal agile response to change on quantitative project performance. However, the positive effects of internal agile response to change, strategic project flexibility and effective engagement on qualitative project performance remain constant under the different levels of project complexity. The findings that emerged from the current study and its recommendations contribute to expanding the knowledge of stakeholder management in the context of complex projects in diverse industries, and they provide practical contributions to enable managers to successfully manage projects. Further, the results provide strong and empirical evidence for project managers, who may benefit from the results by diagnosing and improving their management skills and enhancing their stakeholder management to improve success rates in the context of complex projects. It should be noted that the findings could be applied to projects in general as well as in the context of complex projects.