宇宙微波背景（英语：CMB, cosmic microwave background，又称3K背景辐射）是宇宙学中“大爆炸”遗留下来的热辐射。在早期的文献中，「宇宙微波背景」称为「宇宙微波背景辐射」（CMBR）或「遗留辐射」，是一种充满整个宇宙的电磁辐射。特征和绝对温标2.725K的黑体辐射相同。频率属于微波范围。宇宙微波背景是宇宙背景辐射之一，为观测宇宙学的基础，因其为宇宙中最古老的光，可追溯至再复合时期。利用传统的光学望远镜，恒星和星系之间的空间（背景）是一片漆黑。然而，利用灵敏的辐射望远镜可发现微弱的背景辉光，且在各个方向上几乎一模一样，与任何恒星，星系或其他对象都毫无关系。这种光的电磁波谱在微波区域最强。1964年美国射电天文学家阿诺·彭齐亚斯和罗伯特·威尔逊偶然发现宇宙微波背景，于1940年代开始研究，并于1978年获得诺贝尔奖。

Cosmic background radiation is electromagnetic radiation from the sky with no discernible source. The origin of this radiation depends on the region of the spectrum that is observed. One component is the cosmic microwave background radiation. This component is redshifted photons that have freely streamed from an epoch when the Universe became transparent for the first time to radiation. Its discovery and detailed observations of its properties are considered one of the major confirmations of the Big Bang. The discovery (by chance in 1965) of the cosmic background radiation suggests that the early universe was dominated by a radiation field, a field of extremely high temperature and pressure.