辅助性原则（priciple of subsidiarity）是欧盟法中的一项重要的基本原则。辅助性这个词来源于拉丁文subsidiarius。其基本含义是，只有在成员国采取行动是不充分的情况下，欧盟才能够介入。

Subsidiarity is a principle of social organization that originated in the Roman Catholic Church. In its most basic formulation, it holds that social and political issues should be dealt with at the most immediate (or local) level that is consistent with their resolution. The Oxford English Dictionary defines subsidiarity as the idea that a central authority should have a subsidiary (that is, a supporting, rather than a subordinate) function, performing only those tasks which cannot be performed effectively at a more immediate or local level. The concept is applicable in the fields of government, political science, neuropsychology, cybernetics, management and in military command (Mission Command). In political theory, the principle of subsidiarity is sometimes viewed as an aspect of the concept of federalism, although the two have no necessary connection. The principle of subsidiarity plays an important role in the political functioning of the European Union concerning the relationship between the EU governing bodies and the member states.