In recent years, partnerships between schools and non-system actors, that is, cross-sector alliances, have become increasingly important in education systems around the world, raising concerns and doubts regarding the influence of non-school actors in educational management and leadership. In these debates, trust is regarded as a key element for the success of cross-sector alliances. Although both trust and cross-sector alliances have been increasingly studied in education research in recent years, an investigation of the role of trust in such relationships is still missing. This article analyzes relational trust in cross-sector alliances in education in order to contribute to further theory building. To this end, it implements a qualitative design based on semi-standardized interviews with 21 individuals responsible for managing an illustrative cross-sector alliance. Interviews are analyzed with the help of an approach based on Grounded Theory. The analyses result in a five-dimensional theory of trust in cross-sector alliances, comprising (1) individual attitude, (2) time, (3) professional affiliation, (4) power, and (5) multiplexity. The theoretical approach developed in this article has practical implications for networked leadership and school principals’ work and training as it provides information regarding the skills and knowledge needed for managing cross-sector alliances.