仙后座A（Cassiopeia A，简称Cas A；相关编号：G111.7-2.1、3C 461射电源）是位于仙后座的强射电源。它是银河系内中已知的最年轻的超新星遗迹，也是天空中除太阳外最强的射电源。在天球的位置为赤经232324，赤纬+58°48'54"（2000.0历元，约在仙后座κ星以北），离地球约11000光年，视直径5角分，亮度为6等但由于太弥散与银河背景影响，未能以肉眼直接看到。

遗迹的射电源是在1947年由英国剑桥的射电天文学家发现，是除太阳以外观测到的最强射电源，现在该遗迹仍在膨胀中，据哈勃望远镜测定其物质以每小时5000万千米的速度向外膨胀，从其X射线谱显示喷流富含硅原子，而相对缺少铁原子，另铁组成的柱状结构与喷流垂直。

Cassiopeia A (Cas A) is a supernova remnant (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately 11,000 light-years (3.4 kpc) away within the Milky Way. The expanding cloud of material left over from the supernova now appears approximately 10 light-years (3 pc) across from Earth's perspective. In wavelengths of visible light, it has been seen with amateur telescopes down to 234mm (9.25 in) with filters.