When the public administration (the first sector) started to feel a need or an absolute necessity of executing the public tasks by non-public entities (NGO sector - the third sector, and business sector the second sector), a need of creating the formal - and sometimes informal - "associations" of organizations descended from these three various sectors. Such activities were firstly identified as cross-sector cooperations (mostly run as informal associations of these organizations) and later as cross-sector partnerships (associations created within a framework of formal regulations). Therefore, the author assumes that both internal and external relational capital is a necessary determinant of creation, functioning and, most of all, the legitimacy of cross-sector partnership existence. The author wants to justify the thesis with the analysis of formal solutions and based on almost 10 years (20092019) of empirical research which was conducted in NGOs and public institutions in Silesia, Lower Silesia and Greater Poland voivodeships, as well as during the work with students. The following research tools have been used: participatory observations, interviews, analysis of literature and Internet sources. The aim of this article is an attempt to identify the relational capital within the cross-sector partnerships. Creation of research conclusions and a postulative layer will undoubtedly enable the realization of the article's goal.