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。

The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology. In older literature, the CMB is also variously known as cosmic microwave background radiation (CMBR) or "relic radiation." The CMB is a cosmic background radiation that is fundamental to observational cosmology because it is the oldest light in the universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. However, a sufficiently sensitive radio telescope shows a faint background glow, almost isotropic, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned the discoverers the 1978 Nobel Prize.