第三部门（英语：The third sector）或又称为志愿部门（英语：Voluntary sector），是一社会学与经济学名词，意指在第一部门（Public sector，或称为公部门）与第二部门（Private Sector，或称为私部门）之外，既非政府单位、又非一般民营企业的事业单位之总称。虽然各国对于第三部门的定义各自不同，但一般来说第三部门单位大都是由政府编列预算或私人企业出资，独立维持经营的事业体。一般常见的社团法人、财团法人、基金会、非政府组织（NGO）或非营利组织（NPO）通常都属于第三部门的范畴，虽然每个第三部门单位成立的背景与营运方式都有不同，但普遍来说第三部门单位通常具有像是以社会公益为目的，与不用缴税等特质。

The voluntary sector or community sector (also non-profit sector or "not-for-profit" sector) is the duty of social activity undertaken by organizations that are not-for-profit and non-governmental. This sector is also called the third sector, in contrast to the public sector and the private sector. Civic sector or social sector are other terms for the sector, emphasizing its relationship to civil society. Given the diversity of organizations that comprise the sector, Peter Frumkin prefers "non-profit and voluntary sector".