This research analyzes cross-sector partnerships as a new and powerful form of collaboration that encourages the development of social innovation practices. The study focuses on business–nonprofit partnerships and evaluates their influence on the nonprofit’s development of innovations, capability building, and performance. Empirical research is based on a survey of a representative sample of 325 Spanish foundations. Structural equation techniques and multisample analysis served to analyze the data. The results show that close relationships based on trust and commitment foster the nonprofit’s development of innovations, although the intensity of this effect depends on the type of firms’ contribution to the partnership.