This study aims to define the role of knowledge in a triad of factors determining effectiveness in Interim Management (IM) projects. The discussion is based on the authors’ research concept, which, in addition to knowledge, also explores the categories of trust and power. A longitudinal study using the empirical-inductive approach was conducted in Poland between 2019 and 2021. It included ten enterprises that implemented IM projects in the studied period. The results presented in this article confirm the importance of the empirically adopted study factors, including the transfer of knowledge between the Interim Manager and the client’s (organisation’s) project team. A significant relationship between the level of knowledge and the levels of trust and power emerges as particularly evident. Research can be continued to verify the authors’ initial findings and include the proposed research tools and entities representing different sectors, management cultures and geographical regions in search of additional variables and their correlations with trust, power and knowledge. The research conclusions may prove applicable to both Interim Managers (IMs) and their clients (organisations). They can be used not only for pre-project planning but also during the IM projects.